Document VCS fields

Document VCS fields

Uses Developers's Reference §6.2.5 for inspiration.

Closes: #654958
[jrnieder@gmail.com:
 - declared repositories should be publicly accessible
 - Vcs-Browser should point to a webapp
 - Vcs-<system> should use the version control system's conventional syntax
 - if multiple branches are used for packaging (e.g., "stable",
   "testing", "sid"), any one of them will do
 - for Vcs-Git, "-b <branch>" can be omitted when the intended branch is the
   default branch
 - list some Vcs-<foo> fields by name in the lists in §5.2 and §5.4
 - declared repositories track development of the Debian source
   package, not just the upstream code
 - Vcs-Browser can be a web interface using any protocol (e.g., HTTPS
   is fine)
 - picking a good branch is optional
 Thanks to Russ Allbery for several improvements to the text.]