Add CONTRIBUTING.md file

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+---
+title: Contributing to {rtables}
+---
+
+We welcome contributions big and small to the ongoing development of the {rtables} package. For most, the best way to contribute to the package is by filing issues for feature requests or bugs that you have encountered. For those who are interested in contributing code to the package, contributions can be made by working on current issues and opening pull requests with code changes. Any help that you are able to provide is greatly appreciated!
+
+---
+
+## Filing Issues
+
+Issues are used to establish a prioritized timeline and track development progress within the package. If there is a new feature that you feel would be enhance the experience of package users, please open a [Feature Request issue](https://github.com/insightsengineering/rtables/issues/new?labels=enhancement&template=feature.yml&title=%5BFeature+Request%5D%3A+%3Ctitle%3E). If you notice a bug in the existing code, please file a [Bug Fix issue](https://github.com/insightsengineering/rtables/issues/new?&labels=bug&template=bug.yml&title=%5BBug%5D%3A+%3Ctitle%3E) with a description of the bug and a [reprex](https://reprex.tidyverse.org/) (reproducible example). Other types of issues (questions, typos you've noticed, improvements to documentation, etc.) can be filed as well. Click [here](https://github.com/insightsengineering/rtables/issues/new/choose) to file a new issue, and [here](https://github.com/insightsengineering/rtables/issues) to see the list of current issues. Please utilize labels wherever possible when creating issues for organization purposes and to narrow down the scope of the work required.
+
+---
+
+## Creating Pull Requests
+
+Development of the {rtables} package relies on an _Issue → Branch → PR → Code Review → Merge_ pipeline facilitated through GitHub. If you are a more experienced programmer interested in contributing to the package code, please begin by filing an issue describing the changes you would like to make. It may be the case that your idea has already been implemented in some way, and the package maintainers can help to determine whether the feature is necessary before you begin development. Whether you are opening an issue or a pull request, the more detailed your description, the easier it will be for package maintainers to help you! To make code changes in the package, please follow the following process.
+
+### Pull Request Process
+
+The {rtables} package is part of the NEST project and utilizes [staged.dependencies](https://github.com/openpharma/staged.dependencies) to ensure to simplify the development process and track upstream and downstream package dependencies. We highly recommend installing and using this package when developing within {rtables}.
+
+#### 1. Create a branch
+
+In order to work on a new pull request, please first create a branch off of `main` upon which you can work and commit changes. To comply with [`staged.dependencies`](https://github.com/openpharma/staged.dependencies) standards, {rtables} uses the following branch naming convention:
+
+`issue#_description_of_issue@target_merge_branch`
+
+For example, `443_refactor_splits@main`. In most cases, the target merge branch is the base (`main`) branch. 
+
+In some cases, a change in {rtables} may first require upstream changes in the {formatters} package. Suppose we have branch `100_update_fmts@main` in {formatters} containing the required upstream changes. Then the branch created in {rtables} would be named as follows for this example: `443_refactor_splits@100_update_fmts@main`. This ensures that the correct branches are checked out when running tests, etc.
+
+For more details on `staged.dependencies` branch naming conventions, [click here](https://github.com/openpharma/staged.dependencies#branch-naming-convention).
+
+#### 2. Code
+
+Work within the {rtables} package to apply your code changes. Avoid combining issues on a single branch - ideally, each branch should be associated with a single issue and be prefixed by the issue number.
+
+For information on the basics of the {rtables} package, please read the package vignettes, which are available [here](https://insightsengineering.github.io/rtables/latest-tag/articles/index.html).
+
+For advanced development work within {rtables}, consider reading through the {rtables} Developer Guide. The Developer Guide can be accessed from the {rtables} site navigation bar, and is listed here for your convenience:
+
+- [Developer Guide: Split Machinery](https://insightsengineering.github.io/rtables/latest-tag/articles/dev-guide/dg_split_machinery.html)
+- [Developer Guide: Tabulation](https://insightsengineering.github.io/rtables/latest-tag/articles/dev-guide/dg_tabulation.html)
+- [Developer Guide: Debugging in {rtables} and Beyond](https://insightsengineering.github.io/rtables/latest-tag/articles/dev-guide/dg_debug_rtables.html)
+- [Developer Guide: Sparse Notes on {rtables} Internals](https://insightsengineering.github.io/rtables/latest-tag/articles/dev-guide/dg_notes.html)
+
+##### Code style
+
+The {rtables} package follows the tidyverse [style guide](https://style.tidyverse.org/index.html) so please adhere to these guidelines in your submitted code. After making changes to a file within the package, you can apply the package styler automatically and check for lint by running the following two lines of code while within the file:
+
+```
+styler:::style_active_file()
+lintr:::addin_lint()
+```
+
+##### Documentation
+
+Package documentation uses [`roxygen2`](https://roxygen2.r-lib.org/index.html). If your contribution requires updates to documentation, ensure that the roxygen comments are updated within the source code file. After updating roxygen documentation, run `devtools::document()` to update the accompanying `.Rd` files (do not update these files by hand!).
+
+##### Tests
+
+To ensure high code coverage, we create tests using the [`testthat`](https://testthat.r-lib.org/) package. In most cases, changes to package code necessitate the addition of one or more tests to ensure that any added features are working as expected and no existing features were broken.
+
+##### NEWS
+
+After making updates to the package, please add a descriptive entry to the NEWS file that reflects your changes. See the [tidyverse style guide](https://style.tidyverse.org/news.html) for guidelines on creating a NEWS entry.
+
+#### 3. Make a Pull Request
+
+Once the previous two steps are complete, you can create a pull request. Indicate in the description which issue is addressed in the pull request, and again utilize labels to help reviewers identify the category of the changes contained within the pull request.
+
+Once your pull request has been created, a series of checks will be automatically triggered, including `R CMD check`, tests/code coverage, auto-documentation, and more. All checks must be passing in order to eventually merge your pull request, and further changes may be required in order to resolve the status of these checks. All pull requests must also be reviewed and approved by at least one of the package maintainers before they can be merged. A review will be automatically requested from several {rtables} maintainers upon creating your pull request. When a maintainer reviews your pull request, please try to address the comments in short order - the {rtables} package is updated on a regular basis and leaving a pull request open too long is likely to result in merge conflicts which create more work for the developer.