[DOC] Document primary features of gateway at top of README.
[DOC] Document primary features of gateway at top of README.

Parkjeahwan/awegeeks,whotooktwarden/gatewayd,whotooktwarden/gatewayd,Parkjeahwan/awegeeks,zealord/gatewayd,crazyquark/gatewayd,zealord/gatewayd,xdv/gatewayd,crazyquark/gatewayd,xdv/gatewayd