doc: Added bucket management commands to ops/crush-map

Describes the CLI for adding and removing buckets, in addition to the
'moving' instructions which were already present. Loosely based on the information at: http://www.sebastien-han.fr/blog/2014/01/13/ceph-managing-crush-with-the-cli/

Signed-off-by: Stephen Jahl <stephenjahl@gmail.com>