Add Documentation for custom implementation

[VaZark]

I have separate start_dt and end_dt fields on a model. It'd be neat to use a single instance of the widget to filter over both fields.

Since this is implementation may vary between projects but the pattern itself is pretty common, it'd great if there were additional docs on how to extend the widget to build a custom implementation.

I found an example here : https://www.dothedev.com/blog/2020/04/05/django-admin-list_filter/

An official one would be appreciated nonetheless