CONTRIBUTING: update for format requirements

CONTRIBUTING.md

@@ -4,7 +4,10 @@ see:
 
 * Contributors will need to have signed the
   [CLA](https://ubuntu.com/legal/contributors/agreement).
-* Prerequisites for running tests locally can be installed with `make aptdeps`.
-* Lint, unit, integration, and API tests should be passing.  To simulate that
-  locally please run `make lint check`.  This is a close approximation to what
-  is run in CI today.
+* Format, lint, unit, integration, and API tests should be passing.
+  * format - either run `make format` or `pre-commit install`.
+    See [pre-commit](https://pre-commit.com/#install) for more details on that
+    tool.
+  * all the rest - `make lint check`.
+    Prerequisites for running these locally can be installed with
+    `make aptdeps`.