readme: added links to additional documentation
readme: added links to additional documentation

This commit adds links to the google cloud platform's documentation on
the custom options used by grpc-gateway to generate your http proxy.
This documentation has information about how path, query, and body
parameters are handled, as well as specific assumptions about the
arguments present for different http methods.

Fixes #212

ilius/grpc-gateway,grpc-ecosystem/grpc-gateway,Lantame/grpc-gateway,tmc/grpc-gateway,improbable-io/grpc-gateway,gengo/grpc-gateway,Lantame/grpc-gateway,tmc/grpc-gateway,ilius/grpc-gateway,ilius/grpc-gateway,improbable-io/grpc-gateway,gengo/grpc-gateway