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.
Stylistically, I'm finding the constantly changing perspective of "we" quite dizzying. Not sure how if there's a general approach through these docs, but perhaps it would be better to phrase things we expect someone configuring an Alaveteli installation to do as "You", rather than "We"?