#27 first take on content about network authentication in carrier billing

@marcoscaceres here is a first take for issue #27

I've added it under "web carrier billing payments" since this is not firefox-os specific.

For Firefox OS specific stuff, it'll probably be good to have @kumar303 or @ferjm comment about why they are exposing MCC/MNC (Mobile Country Code/Mobile Network Code) and in which cases they are using it.

https://bugzilla.mozilla.org/show_bug.cgi?id=892068