Document GitHub/Heroku setup for legacy code project phase

[pconrad]

In the legacy code project phase, we typically impose branch protection rules that require an administrator of the GitHub org to approve merge requests.

This is because this is the "gate" where we do grading and give credit, and because for "real legacy code", we are trying to keep the code base up to certain standards.

These are unenforceable if the students have admin access to the repo.

So we set up the repos with the students having all permissions possible short of admin access.

For Heroku, that means that even if the students set up the Heroku deployments, the staff has to actually be the one to link the Heroku deployment to the repo.