added contributing.md doc

CONTRIBUTING.md

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+# Contributing
+
+Contributions are welcome, and they are greatly appreciated! Every
+little bit helps, and credit will always be given.
+
+## Types of Contributions
+
+### Report Bugs
+
+If you are reporting a bug, please include:
+
+-   Your operating system name and version.
+-   Any details about your local setup that might be helpful in
+    troubleshooting.
+-   Detailed steps to reproduce the bug.
+
+### Fix Bugs
+
+Look through the GitHub issues for bugs. Anything tagged with "bug" and
+"help wanted" is open to whoever wants to implement it.
+
+### Implement Features
+
+Look through the GitHub issues for features. Anything tagged with
+"enhancement" and "help wanted" is open to whoever wants to implement
+it.
+
+### Write Documentation
+
+You can never have enough documentation! Please feel free to contribute
+to any part of the documentation, such as the official docs, docstrings,
+or even on the web in blog posts, articles, and such.
+
+### Submit Feedback
+
+If you are proposing a feature:
+
+-   Explain in detail how it would work.
+-   Keep the scope as narrow as possible, to make it easier to
+    implement.
+-   Remember that this is a volunteer-driven project, and that
+    contributions are welcome :)
+
+## Get Started!
+
+Ready to contribute? Here's how to set up for local development.
+
+1.  Fork the repo on GitHub.
+
+2.  Clone your fork locally.
+
+3.  Use `git` (or similar) to create a branch for local development and
+    make your changes:
+
+    ``` console
+    $ git checkout -b name-of-your-bugfix-or-feature
+    ```
+
+4.  When you're done making changes, check that your changes conform to
+    any code formatting requirements and pass any tests.
+
+5.  Commit your changes and open a pull request.
+
+## Pull Request Guidelines
+
+Before you submit a pull request, check that it meets these guidelines:
+
+1.  The pull request should include additional tests if appropriate.
+2.  If the pull request adds functionality, the docs should be updated.
+3.  The pull request should work for all currently supported operating
+    systems and versions of Python.
+
+## Code of Conduct
+
+Please note that the project is released with a Code of Conduct. By
+contributing to this project you agree to abide by its terms.