Improve CONTRIBUTING document

[Belco90]

Some ideas for improving our CONTRIBUTING.md with:

• some AST info
• some link to astexplorer.net as a tool to deal with AST
• new section about how to try changes locally

Feedback from @zaicevas:

For the first 30 minutes I had no idea where to start, until I've realized it's all based on AST and found https://astexplorer.net/
After that it all started to make sense and the code proved to be very clear & extendable. Perhaps it could be useful mentioning this or other similar tools in https://github.com/testing-library/eslint-plugin-testing-library/blob/main/CONTRIBUTING.md

Another that I think could be of use in CONTRIBUTING.md is a section on how to try out changes locally (besides tests). I suppose symlinking with yarn link or npm link is relatively simple and could work here.

Originally posted by @zaicevas in #352 (comment)

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