Added some docs about stop method

Tell about the fact that the stop metod must
return a value which can be evaluated to true if
everything went well.

README.rdoc

@@ -125,7 +125,9 @@ lib/guard/guard-name.rb inherit from guard/guard and should overwrite at least o
           true
         end
         
-        # Call with Ctrl-C signal (when Guard quit)
+        # Call with Ctrl-C signal (when Guard quit).
+        # This method must return a true value 
+        # if everything went well or guard will not stop.
         def stop
           true
         end

lib/guard/guard.rb

@@ -30,6 +30,7 @@ module Guard
       true
     end
     
+    # Retrieve a true value if the instance successfuly stopped
     def stop
       true
     end