doc: add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+This document outlines a few tips for contributing to packse.
+
+## Running tests
+
+packse uses `pytest`:
+
+```
+poetry run pytest
+```
+
+## Updating snapshots
+
+If you make changes to the code that results in a snapshot test failing, then
+you should examine whether the changes are correct. If so, you can re-run the
+tests with the `--snapshot-update` flag:
+
+```
+poetry run pytest --snapshot-update
+```
+
+And then commit the results. In at least some cases, this may commit a snapshot
+that is inconsistent with what CI expects. In this case, you'll want to
+manually back out the change. See [this comment][index-incorrect-snapshot] for
+an example.
+
+[index-incorrect-snapshot]: https://github.com/astral-sh/packse/pull/175#issuecomment-2056964089