added url comment hint on simple push location

> If there is just one option, then I'm all for it.

well, I think @edewit covered all choices:
* Openshift (our cartridge)
* Mozilla's network URL: used by FirefoxOS (no real way to change it, unless devs use our polyfil.. ); JS clients from AeroGear-SimplePush.js could use that too, even if the UPS is running on OpenShift 
* custom server: e.g. one is running ours somewhere else; an option to build Mozilla's Go-based server and run somewhere else is an option as well