README: Add anchors for rules and config options

[jtojnar]

This will allow us to link to them directly.

Using the syntax from https://sublime-and-sphinx-guide.readthedocs.io/en/latest/references.html#use-a-custom-anchor

Example: https://github.com/jtojnar/flake8-bugbear/tree/readme-anchors#user-content-b950