Add section on insecure-registry

Wasted a whole bunch of time dealing with VPN and other issues, but the real issue was this.
+1 for this PR. Causing pain for a few people at the moment. Slightly odd to put a comment though here as a step - how about:

```shell
echo 'EXTRA_ARGS="--insecure-registry <YOUR INSECURE HOST>"' | sudo tee -a /var/lib/boot2docker/profile
```