Add project-agnostic (or templated) instructions on contributing to our tools

From: diffoscope!5 (comment 29261)

moving templated stuff to our website to be honest and link to that. Much more likely that one will invest time in making it better when you are not copy-pasting it into 7 different places that will get out of sync.

nb. It could literally be templated in the sense that we can easily auto-generate (eg.) 7 different pages with mostly-identical content.