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.
Also suggest to add something to the effect that charm proof tries to programmatically ensure the charm is adhering to [Charm Store Policy](https://juju.ubuntu.com/docs/authors-charm-policy.html).