Document how to set up a local build environment with sbuild

For anyone new to the team, who wants to build the Xen packages, we should have a bit of a guide that shows how to set up things.

What we could do is take a team session together, and start with an 'empty' Debian system (any random vm or whatever) and then do the steps from 0 to having Xen debs as output. Then on the go, we document the steps we take, (and links for RTFM etc).

Things like sbuild-createchroot sbuild-debian-developer-setup and https://wiki.debian.org/sbuild will be flying by.

Now, if someone else wants to do the same, and is following the same steps, we have a common understanding, and as existing team we're better able to help.