Add some doc for simple_switch architecture (#218)
Add some doc for simple_switch architecture (#218)

This document mostly details the intrinsic metadata fields supported by
simple_switch.
[skip ci]
hanw/behavioral-model,hanw/behavioral-model,p4lang/behavioral-model,p4lang/behavioral-model,hanw/behavioral-model,p4lang/behavioral-model,hanw/behavioral-model,p4lang/behavioral-model,hanw/behavioral-model,p4lang/behavioral-model