Readme should explain public IP requirements

[strtdusty]

I believe that the process for scanning with clair-scanner is something like

1. clair-scanner starts a local registry
2. clair-scanner calls clair requesting a scan using the --IP parameter
3. clair calls back to the public IP sent and pulls the image

I think this should really be called out in the readme as a major decision in determining if you should use the tool. I was evaluating this for use with my CI build tool but would not be able to open public access back to the build agent.

I can create a PR to update the readme for this if my understanding is correct.