Add CONTRIBUTING.md (#6)

CONTRIBUTING.md

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+# Contributing
+
+This page includes some guidelines to enable you to contribute to the project.
+
+## Found a bug?
+
+If you find a bug in the source code or in using the theme, you can
+open an issue on GitHub.
+Even better, you can submit a pull request with a fix.
+
+## Submission guidelines
+
+### Submitting an issue
+
+Before you submit an issue, please search the issue tracker, maybe an issue
+for your problem already exists and the discussion might inform you of workarounds
+readily available.
+
+We want to fix all the issues as soon as possible, but before fixing a bug we
+need to reproduce and confirm it. In order to reproduce bugs we will need as
+much information as possible, and preferably a sample demonstrating the issue.
+
+### Submitting a pull request (PR)
+
+If you wish to contribute to the code base, please open a pull request by
+following GitHub's guidelines.
+
+## Development Conventions
+
+`atlannot` uses:
+- Black for formatting code
+- Flake8 for linting code
+- PyDocStyle for checking docstrings