Document branch movement procedures

[bryevdv]

Currently back- and forward-porting between branches is a new practice for us. I have basically been winging it, and as a result have forgotten to turn off squash merge a few times. Would like to codify the best practice in a checklist that can be followed while going through the process (that is very loud about the correct kind of merge)

It's possible there is automation that might help here too. Some possible ideas

• A script to make the PR with a loud [NO SQUASH] automatically in the title
• Or that automatically adds a nosquash label (and perhaps we can even actively prevent squash merges when the label is present)
• Or forego PRs altogether, have a script that cherry-picks and simply pushes the commits to the relevant branch

@mattpap regarding the PRs last night, I am not sure it's worth reverting and re-merging, and I don't want to force push again either. I think we just live with the slight wart on the history this time.