Add mbaynton/SimplePuppetProvisioner to list of webhook listeners

[mbaynton]

A modest documentation update ;)

I think it's worth a mention because it's simpler to deploy than alternatives that may require web servers and application frameworks, redis etc to be set up, and because it has other functionality like client certificate signing via http API that may be interesting to some users.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[binford2k]

Invoking the trivial patch exemption for docs update.