Add a small 'Extra Credit' section to the end of Suggestotron

Author: tjgrathwell

sites/curriculum/credits_and_next_steps.step

@@ -2,6 +2,14 @@ message <<MARKDOWN
 Guess what?  _**You're done!!!**_ Congratulations, you just \"finished\" your first rails app!
 
 (They're never _really_ ever finished... have fun tweaking it!)
+
+# Extra Credit
+
+If you got all the way through Suggestotron with some time to spare, here's some extra stuff you can try:
+
+* Add a downvote button that does the opposite of what the upvote button does
+* Sort topics by their number of votes
+
 MARKDOWN
 
 h1 "Authors"