Document `master-pytest.yml` setup instructions

[zanebclark]

Document master-pytest.yml setup instructions in the contributing documentation:

1. Right now, an enterprise account is required to run this script strictly because of the search optimization references. Can we remove those given that they don't have any bearing on schemachange's core functionality?
2. Which repo secrets need to be created?
3. Which scripts need to be run and in which order?