ceph: document shared files in README

#

# The following files are shared by (and manually synchronized

# between) the Ceph userland and kernel client.

#

# userland                  kernel

src/include/ceph_fs.h	    fs/ceph/ceph_fs.h

src/include/ceph_fs.cc	    fs/ceph/ceph_fs.c

src/include/msgr.h	    fs/ceph/msgr.h

src/include/rados.h	    fs/ceph/rados.h

src/include/ceph_strings.cc fs/ceph/ceph_strings.c

src/include/ceph_frag.h	    fs/ceph/ceph_frag.h

src/include/ceph_frag.cc    fs/ceph/ceph_frag.c

src/crush/crush.c	    fs/ceph/crush/crush.c

src/crush/crush.h	    fs/ceph/crush/crush.h

src/crush/mapper.c	    fs/ceph/crush/mapper.c

src/crush/mapper.h	    fs/ceph/crush/mapper.h

src/crush/hash.h	    fs/ceph/crush/hash.h