add section on using monadic Happy with Alex lexer

From the existing documentation, I had a hard time figuring out how to
use the Happy monad directive with a monadic Alex lexer. This section
contains a skeleton example of how to do the combination, along with
how the combined lexer and parser are called by client code.
Merge pull request #25 from acfoltzer/sec-monad-alex

Add section on using monadic Happy with Alex lexer