Added links to resource

Author: Ehsan70

Diff for: simple_switch_stp_13.py

@@ -1,4 +1,9 @@
 # coding=utf-8
+"""
+Note: s lot of the info is derived from the below link:
+http://osrg.github.io/ryu-book/en/html/spanning_tree.html#executing-the-ryu-application
+"""
+
 from ryu.base import app_manager
 from ryu.controller import ofp_event
 from ryu.controller.handler import CONFIG_DISPATCHER, MAIN_DISPATCHER