update readme to use bin/hubot correctly

otherwise users have to resort to installing coffee-script globally
for example #102

README.md

@@ -28,7 +28,7 @@ Install all of the required dependencies by running `npm install`.
 
 It's easy to test scripts locally with an interactive shell:
 
-    % bin/hubot
+    % PATH="node_modules/.bin:$PATH" bin/hubot
 
 ...and to run tests: