Expose the gRPC code generation plugin.

Clarify the unsupported nature of this interface in the README.

Fixes #147.

README.md

@@ -191,3 +191,9 @@ the `plugins` parameter to protoc-gen-go; the usual way is to insert it into
 the --go_out argument to protoc:
 
 	protoc --go_out=plugins=grpc:. *.proto
+
+## Plugins ##
+
+The `protoc-gen-go/generator` package exposes a plugin interface,
+which is used by the gRPC code generation. This interface is not
+supported and is subject to incompatible changes without notice.

protoc-gen-go/link_grpc.go

@@ -31,4 +31,4 @@
 
 package main
 
-import _ "github.com/golang/protobuf/protoc-gen-go/internal/grpc"
+import _ "github.com/golang/protobuf/protoc-gen-go/grpc"