update contributing guide

Author: blink1073

CONTRIBUTING.md

@@ -246,6 +246,18 @@ you are attempting to validate new spec tests in PyMongo.
 
 Follow the [Python Driver Release Process Wiki](https://wiki.corp.mongodb.com/display/DRIVERS/Python+Driver+Release+Process).
 
+## Asyncio considerations
+
+PyMongo adds asyncio capability by modifying the source files in `*/asynchronous` to `*/synchronous` using
+[unasync](https://github.com/python-trio/unasync/) and some custom transforms.
+
+Where possible, edit the code in `*/asynchronous/*.py` and not the synchronous files.
+You can run `pre-commit run --all-files synchro` before running tests if you are testing synchronous code.
+
+To prevent the `synchro` hook from accidentally overwriting code, it first checks to see whether a sync version
+of a file is changing and not its async counterpart, and will fail.
+In the unlikely scenario that you want to override this behavior, first export `OVERRIDE_SYNCHRO_CHECK=1`.
+
 ## Converting a test to async
 The `tools/convert_test_to_async.py` script takes in an existing synchronous test file and outputs a
 partially-converted asynchronous version of the same name to the `test/asynchronous` directory.