Added instructions for using proto files from google/protobuf (#371)

Fixes #370

Readme.md

@@ -105,6 +105,16 @@ To generate the code, you also need to set the include path properly.
 
     protoc -I=. -I=$GOPATH/src -I=$GOPATH/src/github.com/gogo/protobuf/protobuf --{binary}_out=. myproto.proto
 
+To use proto files from "google/protobuf" you need to add additional args to protoc.
+
+    protoc -I=. -I=$GOPATH/src -I=$GOPATH/src/github.com/gogo/protobuf/protobuf --{binary}_out=\
+    Mgoogle/protobuf/any.proto=github.com/gogo/protobuf/types,\
+    Mgoogle/protobuf/duration.proto=github.com/gogo/protobuf/types,\
+    Mgoogle/protobuf/struct.proto=github.com/gogo/protobuf/types,\
+    Mgoogle/protobuf/timestamp.proto=github.com/gogo/protobuf/types,\
+    Mgoogle/protobuf/wrappers.proto=github.com/gogo/protobuf/types:. \
+    myproto.proto
+
 Note that in the protoc command, {binary} does not contain the initial prefix of "protoc-gen".
 
 ### Most Speed and most customization