Documentation request: How to create a custom grammar

[etaiklein]

I want to allow trailing underscores in email addresses for me.com. Ex: '[email protected]'

The docs mention that I can add a custom grammar a plugin, but I'm really not sure where to start.

Currently the section in the user guide reads:

Custom grammar can be added by adding a plugin for the specific ESP to the flanker/addresslib/plugins directory. Then update flanker/addresslib/init.py to include the MX pattern for the ESP you wish to add and add it to the CUSTOM_GRAMMAR_LIST.

I believe this is referring to python packaging plugins. I found these instructions also difficult to follow.

Guiding questions: After pip installing flanker, should I create a plugin.py like these? Where should I put my custom plugin file? How do I install my new plugin? Do I need to fork the repo?

My ask is for clear and comprehensive instructions/examples for adding a custom grammar with screenshots.