Added FAQ for lazy fetch of configuration variables from stage files

Added an FAQ entry about lazily loading configuration variables defined in a deploy/<stage>.rb file in the main deploy.rb file.

Though it probably should have been obvious to me (or at least intuitive) it didn't dawn on me to use a lambda do to this until after reading through the Capistrano source code.  Hopefully this will help someone else.

I didn't see an an obvious way to build this to ensure it looked okay other then it looked okay in my markdown editor.  Feel free to accept it, change it, ask for changes, etc...  I'm probably not the best documentation writer...