Add 'username' parameter for IRC identity to config template

This documents the added functionality from errbotio#256

errbot/config-template.py

@@ -79,9 +79,9 @@
     # 'username' : 'errbot',
     # 'password' : 'changeme',
 
-
     ## IRC mode (Comment the others above if using this mode)
     # 'nickname' : 'err-chatbot',
+    # 'username' : 'err-chatbot',    # optional, defaults to nickname if omitted
     # 'password' : None,             # optional
     # 'server' : 'irc.freenode.net',
     # 'port': 6667,                  # optional