Improve documentation around Python plugins

[ianbamforth]

It would be great if the documentation explained how to use python plugins. AFAICT, you have to add python to the package list (which will install python-dev or similar), and add a switch --enable-python to the configure command, and then you can start adding to the python_plugins mash. I'm yet to figure out if there's an easy way to include a .py file for the module to use either.

Apologies if I've missed something or if this should have been more obvious!