geek/tos3_migration: add notes for upgrade

This fixes #67.

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.