Improve generation README docs
Improve generation README docs

Use paths=source_relative in README

   Probably the most useful feature of protoc-gen-go,
   it's also supported by the gateway generator, so lets
   add it to the example.

Update generation guide

   Instead of using the GOPATH to import the third_party
   folder, recommend the use of of a protobuf source
   tree with the dependencies copied in.

improbable-io/grpc-gateway,grpc-ecosystem/grpc-gateway,improbable-io/grpc-gateway,gengo/grpc-gateway,gengo/grpc-gateway