Require black and isort for contributions (#3329)

* Add explanation to CONTRIBUTNG.md
* Add sample pre-commit script
* Check for correctly formatted files in CI

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

CONTRIBUTING.md

@@ -15,6 +15,7 @@ are important to the project's success.
       but [contact us](#discussion) before starting significant work.
     * Create your stubs [conforming to the coding style](#stub-file-coding-style).
     * Make sure your tests pass cleanly on `mypy`, `pytype`, and `flake8`.
+    * Reformat your stubs with `black` and `isort`.
 4. [Submit your changes](#submitting-changes) by opening a pull request.
 5. You can expect a reply within a few days:
     * Diffs are merged when considered ready by the core team.
@@ -222,8 +223,14 @@ rule is that they should be as concise as possible.  Specifically:
 * use variable annotations instead of type comments, even for stubs
   that target older versions of Python;
 * for arguments with a type and a default, use spaces around the `=`.
-The code formatter [black](https://github.com/psf/black) will format
-stubs according to this standard.
+
+Stubs should be reformatted with the formatters
+[black](https://github.com/psf/black) and
+[isort](https://github.com/timothycrosley/isort) before submission.
+These formatters are included in typeshed's `requirements-tests-py3.txt` file.
+A sample `pre-commit` file is included in the typeshed repository.  Copy it
+to `.git/hooks` and adjust the path to your virtual environment's `bin`
+directory to automatically reformat stubs before commit.
 
 Stub files should only contain information necessary for the type
 checker, and leave out unnecessary detail: