Final call: add github guidelines doc

Hey folks,
Not a rule update this time around. I've added a guide on how to use GitHub
within the CG. Please have a read if you haven't already. The final call
ends in 2 weeks, May 8th, so if you have any comments, please leave a
review before that time.

https://github.com/act-rules/act-rules.github.io/pull/1242

Kind regards

-- 
*Wilco Fiers*
Axe for Web product owner - Co-facilitator WCAG-ACT - Chair ACT-R