Add contributing guidelines

CONTRIBUTING.md

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+# Contributing to `otp_builds`
+
+1. [License](#license)
+1. [Reporting a bug](#reporting-a-bug)
+1. [Requesting or implementing a feature](#requesting-or-implementing-a-feature)
+1. [Submitting your changes](#submitting-your-changes)
+   1. [Code Style](#code-style)
+   1. [Committing your changes](#committing-your-changes)
+   1. [Pull requests and branching](#pull-requests-and-branching)
+
+## License
+
+`otp_builds` is licensed under [Apache License Version 2.0](LICENSE.md) License, for all code.
+
+## Reporting a bug
+
+`otp_builds` is not perfect software and will be buggy.
+
+Bugs can be reported via
+[GitHub issues: bug report](https://github.com/erlef/otp_builds/issues/new?template=bug_report.md).
+
+Some contributors and maintainers may be unpaid developers working on `otp_builds`, in
+their own time, with limited resources. We ask for respect and understanding, and will provide the
+same back.
+
+## Requesting or implementing a feature
+
+Before requesting or implementing a new feature, do the following:
+
+- search, in existing [issues](https://github.com/erlef/otp_builds/issues) (open or closed), whether
+the feature might already be in the works, or has already been rejected,
+- make sure you're using the latest software release (or even the latest code, if you're going for
+_bleeding edge_).
+
+If this is done, open up a
+[GitHub issues: feature request](https://github.com/erlef/otp_builds/issues/new?template=feature_request.md).
+
+We may discuss details with you regarding the implementation, and its inclusion within the project.
+
+We try to have as many of `otp_builds`' features tested as possible. Everything that a
+user can do, and is repeatable in any way, should be tested, to guarantee backwards compatible.
+
+## Submitting your changes
+
+### Code Style
+
+- do not introduce trailing whitespace
+- indentation is 2 spaces, not tabs
+- try not to introduce lines longer than 100 characters
+- write small functions whenever possible, and use descriptive names for functions and variables
+- comment tricky or non-obvious decisions made to explain their rationale
+
+### Committing your changes
+
+Merging to the `main` branch will usually be preceded by a squash.
+
+While it's Ok (and expected) your commit messages relate to why a given change was made, be aware
+that the final commit (the merge one) will be the issue title, so it's important it is as specific
+as possible. This will also help eventual automated changelog generation.
+
+### Pull requests and branching
+
+All fixes to `otp_builds` end up requiring a +1 from one or more of the project's maintainers.
+
+During the review process, you may be asked to correct or edit a few things before a final rebase
+to merge things. Do send edits as individual commits to allow for gradual and partial reviews to be
+done by reviewers.
+
+### What checks do you perform
+
+Check the `.github/workflows` elements for the checks we perform, if you want to do them locally.
+Otherwise you can also push directly to GitHub and wait for CI results and human reviews.