docs: add contributor docs for Python test snapshots

CONTRIBUTING.md

@@ -159,3 +159,21 @@ You must commit the generated files as well as part of your PR.
 
 The `*_extender` files under `docs/reference/cli` are meant as extensions of the autogenerated documentation.
 Those are meant to be updated manually.
+
+## Python test snapshots
+
+Snapshots can be included in Python tests using `inline-snapshot`, following the pattern in
+https://github.com/prefix-dev/pixi/blob/main/tests/integration_python/test_import.py.
+
+1. To add a new snapshot, you first write `assert value == snapshot()`, where `value` is the
+string you are testing.
+2. You can then run your tests with `pixi run test-specific-test your_test_name`. The test should
+error (this is expected), but also create a snapshot inline (inside the parentheses which follow
+`snapshot`), unless the test fails for another reason before hitting the `snapshot()` line.
+If the test fails for another reason, fix that reason and run the tests again.
+3. Running tests in the future will prompt you whenever actual values do not match the snapshots.
+If you want to fix all snapshots and review the diff afterwards, you can use `pixi run snapshot-update your_test_name`.
+4. You may wish to redact parts of snapshots. This will be necessary when values are dependent on
+the machine running the tests, e.g. manifests created with `pixi init`. This can be done using
+`dirty-equals`: see documentation at https://15r10nk.github.io/inline-snapshot/0.23/eq_snapshot/#dirty-equals
+and examples in https://github.com/prefix-dev/pixi/blob/main/tests/integration_python/test_import.py.