[simulators.gateway] Add imitator 'spec' markdown
[simulators.gateway] Add imitator 'spec' markdown

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