nginx.md: Add note about potential security isues

I thought about this while setting this up, and then found this guide (I was setting it up without the guide first.)

The potential security implications are important, so I think we should mention them here on this web page. (We could even go further by outright _warning_ people about this, but perhaps letting people know about it so they can make an informed decision is a better way to go. This can be perfectly fine for certain intranet scenarios.)

docs/recipes/nginx.md

@@ -38,6 +38,12 @@ you want through the secondary authentication mechanism implemented inside your
 proxy, it also requires that you move TLS termination from the Registry to the
 proxy itself.
 
+> Another important thing to note is that by binding your registry to
+> `localhost:5000` without authentication, you open up a potential loophole in
+> your Docker Registry security - anyone who can log on to the server where your
+> Docker Registry is running can push images to your registry, without
+> authentication. This could have potentially devastating effects.
+
 Furthermore, introducing an extra http layer in your communication pipeline
 makes it more complex to deploy, maintain, and debug. Make sure the extra
 complexity is required.