URI docs and updated index

We may want more content on this. Basically, the `host` and `port` is needed if your Hadoop configuration files aren't on the classpath. So this can happen if your application is running outside the cluster and you haven't taken the step of deploying client configuration files to the server running the application. It's also worth being explicit that `host` is the hostname for the namenode and `port` is the port for the namenode (typically `8020`). If you have an HA configuration, you always need client configuration files as the there is no single hostname for the namenode.