code_review.md

We have a fairly good guide to Code Reviews, but can be improved
by adding a few examples of what a good code review looks like
at GitLab, specifically ones where there is a bit of back and
forth, "nit-picking," etc. This would:

+ help set expectations of newly hired engineers around what our
process looks like when it is functioning what level of scrutiny
their code will be under

+ how we have technical conversations

+ show by example how after you're done crafting a solution, there
can still be extra work done either tidying up code and/or managing
the communication and conversations about your proposed MR