Update listing API documentation
Update listing API documentation

Add /ob/listings API call and update the return to /ipns/{peerId}/listings/{slug}.json

OpenBazaar/openbazaar-go,duomarket/openbazaar-test-nodes,jackkleeman/openbazaar-go,hoffmabc/openbazaar-go,gubatron/openbazaar-go,JustinDrake/openbazaar-go,yurizhykin/openbazaar-go,gubatron/openbazaar-go,duomarket/openbazaar-test-nodes,hoffmabc/openbazaar-go,JustinDrake/openbazaar-go,OpenBazaar/openbazaar-go,JustinDrake/openbazaar-go,cpacia/openbazaar-go,gubatron/openbazaar-go,duomarket/openbazaar-test-nodes,jackkleeman/openbazaar-go,yurizhykin/openbazaar-go,yurizhykin/openbazaar-go,OpenBazaar/openbazaar-go,jackkleeman/openbazaar-go,hoffmabc/openbazaar-go,cpacia/openbazaar-go,cpacia/openbazaar-go