add the XML-RPC endpoint URL to the docs

[domnikl]

I had to read the code in order to get the endpoint's URL, would be nicer if there were a small passage in the docs where users could get the endpoints URL from :)

[mnaberez]

http://supervisord.org/introduction.html

The same HTTP server which serves the web UI serves up an XML-RPC interface that can be used to
interrogate and control supervisor and the programs it runs. To use the XML-RPC interface, connect to
supervisor’s http port with any XML-RPC client library and run commands against it. An example of doing
this using Python’s xmlrpclib client library is as follows.

import xmlrpclib
server = xmlrpclib.Server('http://localhost:9001/RPC2')

[domnikl]

Sorry, didn't assumed it to be in the introduction ... assumed it to be on the api page http://supervisord.org/api.html

[mnaberez]

That makes more sense. We should either move it there or put a link to it on that page.

I'll leave this issue open until we do that.