[ttnctl] Add Quick Start Guide to RootCmd
[ttnctl] Add Quick Start Guide to RootCmd

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