Add contribution guide to README

Start a little contribution guide in the README. Mention that this
project uses pre-commit hooks and describe how to install them.

README.md

@@ -208,3 +208,18 @@ This is the Base64 encoding of the following JSON document:
 ```json
 {"identity": {"account_number": "0000001", "internal": {"org_id": "000001"}}}
 ```
+
+## Contributing
+
+This repository uses [pre-commit](https://pre-commit.com) to check and enforce code style. It uses
+[Black](https://github.com/psf/black) to reformat the Python code and [Flake8](http://flake8.pycqa.org) to check it
+afterwards. Other formats and text files are linted as well.
+
+Install pre-commit hooks to your local repository by running:
+
+```bash
+$ pre-commit install
+```
+
+After that, all your commited files will be linted. If the checks don’t succeed, the commit will be rejected. Please
+make sure all checks pass before submitting a pull request. Thanks!