docs: add brief contributing and contact docs

To accompany live-manual MR # 27...

Per discussion in live-manual MR # 27, with text such as coding style preferences being moved out of the manual content itself, it is preferable to have a file in each of the live-build, live-boot and live-config repos to refer users to where they can find the common such content.

An additional 'contact' file is added also with basic contact details, based upon those in the manual content. (Optional, just though it might be useful).