Add Sponsor button to GitHub

[PatReynolds]

Include notice re acknowledgement.

E2 S

[PatReynoldsFUG]

Turns out to be a developer task.
See https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/displaying-a-sponsor-button-in-your-repository

The option we need to implement, I think is:

custom: # Replace with up to 4 custom sponsorship URLs e.g., ['link1', 'link2']

For discussion: link to https://www.freeukgenealogy.org.uk/help-us-keep-history-free/how-to-donate/ or to the links to By Card, By DD, By PP and ByPP$?

Include notice re acknowledgement: did this mean Code of Fundraising Practice?