docs/customizingyourgateway: add ?pretty example (#954)
docs/customizingyourgateway: add ?pretty example (#954)

As discussed in slack. The code is rough, but should illustrate the
point.

Signed-off-by: Stephan Renatus <3d1a7d0a294b6d8e799ad6e122143905f17f9530@chef.io>
improbable-io/grpc-gateway,gengo/grpc-gateway,gengo/grpc-gateway,improbable-io/grpc-gateway,grpc-ecosystem/grpc-gateway