Add grpc example to protoc_plugin README (#514)

protoc_plugin/README.md

diff --git a/protoc_plugin/README.md b/protoc_plugin/README.md
index 40996fd..20506d3 100644
--- a/protoc_plugin/README.md
+++ b/protoc_plugin/README.md
@@ -82,6 +82,14 @@
 options `<option 1>` and `<option 2>` like this:
 
     --dart_out="<option 1>,<option 2>:."
+    
+### Generating grpc Headers
+
+To generate code for [grpc], you will need to pass in the `grpc` option:
+
+    --dart_out="grpc:."
+
+[grpc]: https://pub.dev/packages/grpc
 
 ### Generating Code Info