engineering: document when to (or not to) introduce a new service

Created by: slimsag

This is an attempt at re-hashing what I had written a little over a year ago in https://github.com/sourcegraph/sourcegraph/pull/5487 with the intent of actually landing/merging it this time :)

Since the previous PR was made, our team has grown and I believe we operate in a more established/principled way in the decisions we make - as such I have tried to rephrase this document in a mostly "here are the negative tradeoffs you might not have thought of" and left the decision ultimately up to the reader - rather than having something more prescriptive.

Please make suggested changes (or direct edits), not comments, so perfect does not end up as the enemy of good and I can merge this :)