Add README.source and a script to do a Git-aware merge of upstream

Add README.source and a script to do a Git-aware merge of upstream

* Add README.source explaining maintenance procedures and a
  debian/import-upstream script to do a Git-aware merge of a new
  upstream release that will allow us to cherry-pick upstream patches
  and do proper merges.

Note that I'm not planning on re-importing the upstream 1.4.11 release
using this new method, so this technique will not be effective in the
master branch until upstream releases 1.4.12.

(cherry picked from commit a12f6de21906ad52211a0dd0d9b923fba204b2fe)