geek/tos3_migration: add notes about upgrade process and branch changes
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.
I've attached a patch with some suggestions, based on my person experience of seeing my branch change etc; feel free to adjust accordingly.