Add clarifying details on cloud config

Add clarifying details on `installing to disk`, specifically with the cloud config extension and naming.

The goal is to make installing CoreOS as simple as possible, its a great operating system and should be easy for anyone to use it. 
Instead of breaking up the flow of the doc with a note, can we explain it as a part of the example workflow? How about:

Jump over to the [docs to learn about the supported features][cloud-config].

The installation script will process your `cloud-config.yaml` file specified with the `-c` flag and place it onto disk. It will be installed to `/var/lib/coreos-install/user_data` and evaluated on every boot. Cloud-config is not the only supported format for this file &mdash; running a script is also available.

A cloud-config that specifies an SSH key for the `core` but doesn't use any other parameters looks like:

```yaml
#cloud-config

ssh_authorized_keys:
  - ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC0g+ZTxC7weoIJLUafOgrm+h...
```

To start the installation script with a reference to our cloud-config file, run:
```
coreos-install -d /dev/sda -C beta -c ~/cloud-config.yaml
```