Extend contributors guide #4485

Following https://github.com/sindresorhus/refined-github/pull/4200

The project could include further documentation: https://github.com/sindresorhus/refined-github/blob/main/collaborators.md

What else should we include? This is different from the generic contributors guide, which is more of an entry point and doesn't need to include a lot of details about the codebase.

This file could include instructions on how the codebase is organized and nitpicks to ensure consistency when reviewing, which currently are "unwritten rules" and lints like:

• how to cache API calls correctly
• sort imports a certain way
• when to use elementReady and what it is
• keep in mind GHE when using absolute URLs
• use // TODO[2021-10-01]: GHE #1234 comments to remind us to drop old code after a year
• GHE support policy: I suggest dropping code a year after it has changed on GitHub.com
• GHE code: preserve unchanged instead of mixing it with new code, unless it's a single selector change. When dropping it, it'll be as easy as deleting whole lines instead of untangling it from live code.

Also in the contributors guide we could add:

• general code instructions (e.g. use this listener instead of that)
• how to reverse engineer GitHub