add CONTRIBUTING.md

.github/CONTRIBUTING.md

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+# Contributing to python-us
+
+Thanks for your interesting in contributing to the project!
+
+Before spending time working on a pull request, please [open an idea discussion](https://github.com/unitedstates/python-us/discussions/new?category=ideas) so we can ensure the change is in line with the goals of the project. Once you've got the go ahead, open a PR with your changes and reference the discussion in the description of the PR.
+
+To speed up the review process, please ensure:
+
+* your change has corresponding tests that run with the existing pytest suite and pass.
+* the code is formatted with [black](https://black.readthedocs.io/en/stable/index.html), included in the dev dependencies.
+* you are open to feedback!
+
+Thank you and we look forward to your contribution! ✨