Initial draft / write-up of Charm Review Process document

This is the initial draft of a Charm Review Process document. This document is meant to outline the process for both ~charmers as well as community charm reviewers. Up until now, there has not been a simple yet detailed process on **how** to review Juju Charms, so it has not provided the community a good path on how to get involved in the overall review process.

Do note, this document is not, as it stands, meant to completely replace the reference-reviewers document, rather to supplement it. The current reference-reviewers document contains a bunch of unnecessary cruft, Ubuntuisms, etc. and will be reviewed, modified, and submitted in a separate pull request.

I look forward to everyone's comments, suggestions, etc. I think we should have a happy middleground that ensures the document is not overly complex, provides community reviewers and charmers alike a clear path on how to review charms, and doesn't lose out on "valuable" information.
Nipick: backtick for `trusty`.