doc: update instructions for RPM distros

Fix RPM building instructions: this has been broken since
libs3 was included inline in the ceph repo as a submodule.
"rpmbuild -tb" was concatenating the ceph.spec and
libs3.spec files, resulting in something that didn't work.

Also, the instructions suggested downloading a .tar.gz file
whereas the specfile requires a .tar.bz2 file.

Also, add a convenient yum command line for getting the compile
dependencies on Fedora 20.

Signed-off-by: John Spray <john.spray@inktank.com>