[Docs]: Add docs for `customValidators`

[KATT]

Is there an existing issue for this?

• [X] I have searched the existing issues and my issue is unique
• [X] My issue appears in the command-line and not only in the text editor

Description Overview

@JoshuaKGoldberg suggested I should post an issue here 🙃

• customValidators are largely undocumented https://github.com/search?q=repo%3Ajsx-eslint%2Feslint-plugin-react+customValidators+language%3AMarkdown&type=code&l=Markdown
• It's mentioned in the docs here and in other places, but I can't figure out how to use it

Expected Behavior

A code example on how to use customVAlidators

eslint-plugin-react version

n/a

eslint version

n/a

node version

n/a

[ljharb]

Looks like it was added in docs here, and to the rule in #160.