Improve developing documentation

[alexandear]

Currently, the development documentation in DEVELOPING.md and the "Writing a Custom Rule" section in README.md is outdated since we introduced the ConfigurableRule interface in #1185. Additionally, the information is slightly duplicated between documents.

What I propose

1. Move everything from "Writing a Custom Rule" to DEVELOPING.md. Add a link in the README that says "See DEVELOPING.md for instructions on how to write a custom rule."
2. Remove make install from DEVELOPING.md.
3. Extend DEVELOPING.md with a section about configurable rules and the ConfigurableRule interface.
4. Add a link to DEVELOPING.md in CONTRIBUTING.md.