Add notes on cleaning up setup.py installs

[PythonCHB]

If setup.py (or pip install) is run, and then the setup.py file is changed, pip can't clean up an old install. And if some of the cruft left behind (like a mailroom.py script in the Scripts folder. it can wreak havoc.

We need to write up some "how to clean things up" notes:

• Finding site-packages and hand deleting things
• Finding the Scripts dir and cleaning it out.

[PythonCHB]

Oh -- and make sure there's a note in there about never having a scripts and entry-points at the same time.