Update HACKING to reflect current state.
Update HACKING to reflect current state.

The HACKING document was out-of-date regarding dependencies, required steps,
etc.  Following the instructions on a newly installed, non-developer machine
showed the places where packages were missing.

R=frankban, teknico
CC=
https://codereview.appspot.com/7030056

CanonicalJS/juju-gui,mitechie/juju-gui,bac/juju-gui,mitechie/juju-gui,bac/juju-gui,bac/juju-gui,CanonicalJS/juju-gui,bac/juju-gui,jrwren/juju-gui,jrwren/juju-gui,jrwren/juju-gui,mitechie/juju-gui,CanonicalJS/juju-gui,jrwren/juju-gui,mitechie/juju-gui