Add redaction guide

- Kept the wording of "General Law" as that's how its referred to in the theme that I link to in this guide.
This document came out of a rough guide I wrote with the intention of handing to re-users as a case study of something we have done. It made more sense to just make it generally available. This guide isn't really a full instruction document as it doesn't cover adding all the extra fields to the theme. I'm :+1:  on the sentiment, but need to give it a bit more thought of what to do about it.