ctlplugin stanza

sadly this doesn't seem to be able to go into the conf.d directory as
supervisorctl doesn't follow includes.

Also sadly, there is no documentation I can find about ctlplugin, and
there's only one mention of that string in the entire repo!
the notify here isn't required as the concat has it
add example of usage to README.md
Merge branch 'supervisorctl_plugin' into 3dna
supervisord::rpcinterface define
Merge branch 'rpcinterface' into 3dna