[GRPC] Rename documentation file

This will make it automatically gets rendered when browsing the repo.

Change-Id: I78bc4207c4ddf61816a9be32600ca2f6a0f3f216
diff --git a/android/android-grpc/docs/grpc-samples/GRPC.MD b/android/android-grpc/docs/grpc-samples/README.MD
similarity index 100%
rename from android/android-grpc/docs/grpc-samples/GRPC.MD
rename to android/android-grpc/docs/grpc-samples/README.MD