Update sample treefile and add repo file as an example

This patch is to update treefile example and add example repo file.

I followed [this steps](http://www.projectatomic.io/blog/2014/08/build-your-own-atomic-centos-or-fedora/)
and tested by `$rpm-ostree compose tree --repo=/srv/rpm-ostree/repo fedora-rawhide-docker.json`