Add CONTRIBUTING.md file

CONTRIBUTING.md

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+# Contributing Guidelines
+
+**Do** follow [the seven rules of a great Git commit message][1].
+
+**Do** follow [the Clojure Style Guide][2].
+
+**Do** include tests for your change when appropriate.
+
+**Do** ensure that the CI checks pass.
+
+**Do** squash the commits in your PR to remove corrections
+irrelevant to the code history, once the PR has been reviewed.
+
+**Do** feel free to pester the project maintainers about the PR if it
+hasn't been responded to. Sometimes notifications can be missed.
+
+**Don't** include more than one feature or fix in a single PR.
+
+**Don't** include changes unrelated to the purpose of the PR. This
+includes changing the project version number, adding lines to the
+`.gitignore` file, or changing the indentation or formatting.
+
+**Don't** open a new PR if changes are requested. Just push to the
+same branch and the PR will be updated.
+
+**Don't** overuse vertical whitespace; avoid multiple sequential blank
+lines.
+
+**Don't** docstring private vars or functions.
+
+[1]: https://chris.beams.io/posts/git-commit/#seven-rules
+[2]: https://github.com/bbatsov/clojure-style-guide