Expose the gRPC code generation plugin.

Clarify the unsupported nature of this interface in the README.

Fixes #147.

README.md

diff --git a/README.md b/README.md
index d48e91a..8fdc89b 100644
--- a/README.md
+++ b/README.md
@@ -191,3 +191,9 @@
 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/grpc/grpc.go

diff --git a/protoc-gen-go/internal/grpc/grpc.go b/protoc-gen-go/grpc/grpc.go
similarity index 100%
rename from protoc-gen-go/internal/grpc/grpc.go
rename to protoc-gen-go/grpc/grpc.go

protoc-gen-go/link_grpc.go

diff --git a/protoc-gen-go/link_grpc.go b/protoc-gen-go/link_grpc.go
index 24e490e..532a550 100644
--- a/protoc-gen-go/link_grpc.go
+++ b/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"