Add "How to add a site" to RTW documentation

[mo-gill]

We need to provide details of how people can add a site to the RTW

There has been discussion about potentially adding a Developer Guide, and the "how to add a site" documentation could be added to this (as well as the "how to add a recipe" documentation).
#3614 (comment)

We should also document which sites RTW works on. We could add a sentence to the about.rst file or alternatively add a new file similar to tested_recipes.rst.