Add code review guidelines for changing running infrastructure

[yuvipanda]

Writing and reviewing terraform code has different challenges than
reviewing other kinds of code. This is a draft attempt at
codifying how code review and merges work with terraform in our
repo, so we can make changes safely and quickly

closes 2i2c-org/team-compass#213

[sgibson91]

As a draft, I think this is a great start. Clearly explains why the code review is complex and how the different scenarios of code review differ with clear process for each 👍🏻 Thank you!

[damianavila]

I left a question but nothing blocking this PR.
Even as a draft it is a great first step and I generally agree with the described workflows!

[choldgraf]

This is helpful context on Terraform + I like the process around it. Thanks for this write-up!

I have two quick comments but they're both related to the same idea around "state of cluster" vs. "state of codebase". Curious what others think, but I don't think either should block this PR

[choldgraf]

I've just pushed some changes to this PR after some of our recent conversations and feedback! Would love another round of reviews from people to see if this looks good to folks!

[choldgraf]

I requested reviews from some more team members, since my push changes and updates the wording a little bit. It would be good if at least one person could give this a 👍 and make sure that we haven't missed anything important.

Also I can't request a review from @yuvipanda since he's the author, but I would love if he could confirm that this PR still looks good to him w/ my changes :-)

[consideRatio]

I made two suggestions, but otherwise this LGTM! Nice work!

[choldgraf]

many thanks for the review @consideRatio - changes accepted!

for others - let's leave this open for another 48 hours. If somebody else approves then I think we can just merge it in unless somebody objects.

[damianavila]

LGTM

[damianavila]

LGTM

Not sure about the CI failure but content LGTM...

[choldgraf]

OK - we've got a few approves, and CI is now happy, and we've given it several days, so I'm gonna merge this one in! Thanks for all the feedback everybody :-)