#53 "What is Middlegen" Docs

[Aslak Hellesřy]

Add a section in the docs that describes the philosophy
behind Middlegen. People tend to have problems
understanding what it is/does. Example:

http://glen.blog-city.com/readblog.cfm?BID=10306

Doco should describe jdbc, data model, velocity,
xdoclet, generic nature of middlegen, plugins... Some
pisctures would be nice to have too.