Tutorial doc to describe on how a user can create their own rule

[ericwb]

Is your feature request related to a problem? Please describe.
A simple to follow document that outlines how a user can create their own rule.

Describe the solution you'd like

• Provide a template of the python code with commented out options
• Instruct how to create a new rule extension via the entry_points
• Need to determine how best to assign rule IDs. Should they be allocated centrally? User defined? Special prefix for user defined? Need to avoid collisions.

Describe alternatives you've considered
n/a

Additional context
n/a

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