Documentation: usage in CI

[simonw]

Since people keep asking for a --check option for CI systems:

• A --check mode would be useful #161
• [Feature Request] Add --check #147
• Support --check flag. #91
• pass --check option to black #42
• --diff option #3

[asottile]

the intended mechanic is to use this via pre-commit in CI which in general "just works" -- I have added your note about the nonzero exit code here: #163