Update Contributing section to include initialization and updating the submodules

Since it's not obvious that edge Sinatra requires its own version of Rack, add
instructions on how to do so. Also include (more) detailed instructions for
first time cloning, updating an existing clone, and using edge Sinatra in an app.

README.rdoc

@@ -449,12 +449,24 @@ Options are:
 
 = Contribute
 
+== First Time: Cloning the sinatra repo
+
   cd where/you/keep/your/projects
   git clone git://github.com/bmizerany/sinatra.git
-  cd your_project
+  cd sinatra
+  git submodule update --init
+  cd path/to/your_project
   ln -s ../sinatra/
+  
+== Updating your existing Sinatra clone
+  
+  cd where/you/keep/sinatra
+  git pull
+  git submodule update --init
+  
+== Using edge Sinatra in your app
 
-at the top of your sinatra.rb file
+at the top of your sinatra_app.rb file:
 
   $:.unshift File.dirname(__FILE__) + '/sinatra/lib'
   require 'sinatra'