Update botexample to call out Python 2-only support

web.py only supports Python 2, therefore this script only supports
Python 2.

Author: cmlccie

examples/botexample.py

@@ -18,6 +18,10 @@
 'SPARK_ACCESS_TOKEN' environment variable on the web server hosting this
 script.
 
+NOTE:  While this script is written to support Python versions 2 and 3, as of
+the time of this writing web.py (v0.38) only supports Python 2.
+Therefore this script only supports Python 2.
+
 """