Add documentation around the custom rules

[ericwb]

Is your feature request related to a problem? Please describe.
The custom rules function needs documentation on creation and usage.

Describe the solution you'd like

• Add a README in the custom directory on how to run with those rules
• Add a markdown doc explaining how to create custom rules and test them

Reference:

• https://tree-sitter.github.io/tree-sitter/using-parsers/queries/1-syntax.html
• https://tree-sitter.github.io/tree-sitter/7-playground.html

Describe alternatives you've considered
n/a

Additional context

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