Document unstable nature of CephFS

- Add note to docs indicating that CephFS is not recommended for
  production datasets.
- Add note to docs indicating that running CephFS with multiple MDS
  servers is not currently recommended.

This fixes issue #5797 http://tracker.ceph.com/issues/5797

Signed-off-by: Scott Devoid <devoid@anl.gov>
Merge pull request #479 from devoid/fix-5797

Document unstable nature of CephFS