Document whether SIM card is needed

[WesleyAC]

It's unclear to me from reading the README whether a SIM card for the Orbic is needed or not to use rayhunter. My assumption is that it is required, since seeing the full range of what traffic could look like would presumably require having valid subscriber information, but it's not 100% clear to me, and I would expect that to be explicitly mentioned in the section that mentions hardware. This matters since the cost and fuss of operating one of these devices is affected by whether a recurring subscription for a SIM card is required for it to remain useful.