geek/tos3_migration: add notes for upgrade process & branch changes

This fixes #67 (closed).

It would be helpful if the 3.x to 5.x documentation explained that my OS branch would be switched to HBK (I was on stable) and likewise knowing what to expect from the upgrade process would help someone going through this for the first time.

This PR is based on my person experience of seeing my branch change and assuming that's expected behaviour at this point in time. Feel free to edit accordingly.