Add 'clean repos' appendix about training-manual

[beardofedu]

@brianamarie does this just need to be added as a file to the appendix, or are there other requirements to provide working instructions for the trainers?

[brianamarie]

does this just need to be added as a file to the appendix, or are there other requirements to provide working instructions for the trainers?

@beardofedu I think an appendix will do, maybe with something at the top clarifying what teachbacks are so those in regular training don't think it applies to them. In TtT I'd be comfortable telling trainers to check out the appendix for info on how to prepare their repositories, and I think they'd know what to do from there.