Feature: README File for /var/spool/cvmfs

This is a small README file installed into `/var/spool/cvmfs` to serve as a quick reference for the files and directories used inside `/var/spool/cvmfs/<FQRN>`. We discussed the addition of such a README file in [this pull request](https://github.com/cvmfs/cvmfs/pull/552).
I decided to install the README file through the usual `cvmfs-server` package though, instead of creating it on the fly.