document PR merge process

[wileyj]

this needs to be documented to prevent PR's from merging that break things.
i.e. if a change is made to how the tests archives are built, maybe require a succesful workflow using that updated composite action.

other things to consider: how many approvals should be required? protected branches?
etc.

[pheobeayo]

can I take up this issue @wileyj ?

[wileyj]

certainly!