Document our guidelines for mocking

[gpsaggese]

          Ugh. There is so much mocking that at best we test only what you already know it works, at worst we don't really test anything.

Our rule for mocking is to mock only the external services (in this case we should mock the interaction with GH)

Can we improve helpers_root/docs/coding/all.write_unit_tests.how_to_guide.md
explaining these, making examples and so on?

You can work with @heanhsok on this

Originally posted by @gpsaggese in #649 (comment)