[simulators.gateway] Add main.go to hold package doc
[simulators.gateway] Add main.go to hold package doc

TheThingsNetwork/ttn,mgranberry/ttn,LoRaWanSoFa/ttn,TheThingsNetwork/ttn,LoRaWanSoFa/ttn,jvanmalder/ttn,TheThingsNetwork/ttn,jvanmalder/ttn,jvanmalder/ttn,mgranberry/ttn,mgranberry/ttn