[main] Doc improvements in file modules/ROOT/pages/storage.adoc

[einarpersson]

Hi,

As a new user to coreos I found/find the storage chapter a bit confusing and overwhelming.

I would like to suggest:

1. Add a brief hello world example of mounting an additional physical disk with existing content. Similar to the nfs network sample.
2. Clarify when it is necessary to specify a storage: disk/filesystem section of the butane config, and when it is not.

In my case I had two disks, one disk to install coreos to and another one with existing content which I want to keep as-is and just mount somewhere. I could not find a example that matched this fairly common setup. After asking in the fedora chat i've understood that I only need to create a systemd mount file (either as storage: .. files: ... in the butane file, or later after booting). I don't need to specify the disk or filesystem in storage:

I understand that stuff like above may be redunant for experienced users, but I hope you can accept my proposal coming from a new user.