Comment CMS PRs against master

I'm really not a fan of repo specific code in the hooks (TODO - Plugin system to do this?), but alas, while it's there, might as well use it.

The CMS stopped accepting PRs against master, and some of us keep merging to master.  So, to make it easier for us and users to realize this, the bot will comment PRs (such as https://github.com/joomla/joomla-cms/pull/2750) notifying of such.