Add documentation describing how to update the plugin protocol

[SarahFrench]

Documenting the process for updating the protocol - thanks to @dbanck for help with the process around symlinks!

Target Release

N/A

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

[jbardin]

I was just contemplating removing this whole mess of paperwork for protocol changes. The idea was that external groups could easily reference a specific point release for the protobbuf without pulling it from VCS, but it's created this whole mess of process and leads itself to either be forgotten or extra versions created, and we have no sane diffs for review.
Maybe we go back to minor releases editing the files in place?

[SarahFrench]

I was just contemplating removing this whole mess of paperwork for protocol changes. The idea was that external groups could easily reference a specific point release for the protobbuf without pulling it from VCS, but it's created this whole mess of process and leads itself to either be forgotten or extra versions created, and we have no sane diffs for review. Maybe we go back to minor releases editing the files in place?

I feel too new to have an opinion. I can definitely see there's risk around accidentally making >1 new minor version per TF minor version and I'm not sure how that could be protected against (could make a GHA?). For now having this PR's write up is useful as a way to make this more accessible with its current process.

[jbardin]

Yeah, the minor version thing can't be protected against, but I really just want regular diffs 😸