Add documentation for how to create a community rule

[mnojek]

Since we want the users to contribute more and the community rules themselves, we need to provide a good instruction that explains this process step-by-step. It should contain instructions:

• what files need to be created and where should be placed
• how to name the rule, provide proper ID and assign proper severity
• how to implement the logic in Python and how to traverse AST tree (especially explain how generic_visit and super().visit_File() work)
• ...?

Maybe we can provide a command to generate a template for a new rule?

This task is part of the #809 issue.