Testing procedure should be documented.

I am sure most people building CMake are going to try to change it. There should be a guide to testing locally that will explain things like which generators get tested by default and how to test a different one. The guide should be easily-found from the prominent build instructions in the README, as well as from testing.rst, source.rst and possibly review.rst. I'm happy to write a first draft myself (it will be very short) if I can get guidance on where to put it. testing.rst seems like an obvious choice from the filename but that is currently framed as being about CI.