Add Code Review guide

Asychronous code review via Github pull requests can be incredibly
efficient, but easy to accidentally offend or miscommunicate if we
aren't careful about our language choices or have a common expectation
of how to act when reviewing and responding to reviews.

This new guide attempts to set some basic expectations to streamline
code reviews and help us stay happy, healthy, and working closely as a
team.

* Distinguish between discussions that are appropriate for code reviews
  versus those that are appropriate for technique discussions.
* Include simplicity as a goal.
* Summarize offline conversation.
* Show, not tell, how to link to project management system ticket.
* Clarify sign off.