added citation

Author: dmalan

src/cs50/flask.py

@@ -21,6 +21,7 @@
         pass
 
     # Add support for Cloud9 proxy so that flask.redirect doesn't redirect from HTTPS to HTTP
+    # http://stackoverflow.com/a/23504684/5156190
     if getenv("C9_HOSTNAME") and not getenv("IDE_OFFLINE"):
         try:
             import flask