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.
I found this to be quite a confusing opening sentence. Perhaps expressing it from their POV, rather than ours might help? [e.g. something like: “In some countries, local requirements mean that requests need to contain personal information, such as the address or ID number of the person asking for information. Usually, requesters do not want this information to be displayed to the general public."]