Docs for "triggers" integration

New section describing the triggers file addition, and related changes to control.

I'm not sure if dh-virtualenv even works with Python3, but this change ties packages to Python2. The "Pre-Depends" on dpkg is from the Debian docs on doing noawait triggers, and I'm not 100% sure the ones on Python are needed, but then they also don't hurt IMHO (except for the Python3 thing).