Reword/rework README

Somewhat better, I'm sure it will need more details eventually.
Add support for renaming hostname.if files

I always want to store my hostname.if files in my siteXX file, but the
installer always overwrites them.  This way I could even detect (somehow)
what the names of the interfaces are someplace.
Add the first example of a config using sxxu

This is a pretty simple example and I have more that I need to setup
configs for but for now, something is better than nothing.
Add an example flashrd role

This is all I needed to do now that the flashrd.site patches are in
flashrd's github repo.

This was as simple as `echo role-flashrd >> common/siteXX/roles`
and it just worked.  (I am sure that some things also needed to be
adjusted for the readonly filesystems but it boots up)