Document that golang's implementation is limited

Author: mika

contrib/golang_impl/README.md

@@ -4,6 +4,10 @@
 
 You need to have docker >= 0.5.0 up and running.
 
+## Limitations
+
+This implementation doesn't provide all the features that are available in the Python implementation of the Docker Registry.
+
 ## Build and start docker image for registry
 
     $ git clone https://github.com/dotcloud/docker-registry.git docker-registry.git
@@ -34,4 +38,4 @@ the tag includes a port.
 
     $ docker run 127.0.0.1/test/test
 
-Now the image is fetched from your local registry and executed. Should print out `world`.
+Now the image is fetched from your local registry and executed. Should print out `world`.