Add a standard responses section to API.md
Add a standard responses section to API.md

MrStrong/Sia,ericflo/Sia,ChrisSchinnerl/Sia,rivine/rivine,rivine/rivine,mharkus/Sia,ChrisSchinnerl/Sia,mtlynch/Sia,mnsl/Sia,MrStrong/Sia,mnsl/Sia,NebulousLabs/Sia,ericflo/Sia,mharkus/Sia,NebulousLabs/Sia,mtlynch/Sia