Add installation/usage instructions

Since you need to do more than just `bundle install` to get it working.

README.rdoc

@@ -35,6 +35,19 @@ You can also use permit on nested parameters, like:
 
 Thanks to Nick Kallen for the permit idea!
 
+== Installation
+
+In Gemfile:
+
+    gem 'strong_parameters'
+
+and then run `bundle`. To activate the strong parameters, you need to include this module in
+every model you want protected.
+
+    class Post < ActiveRecord::Base
+      include ActiveModel::ForbiddenAttributesProtection
+    end
+
 == Todos
 
 * Automatically permit parameters coming from a signed form [Yehuda]