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.

Signed-off-by: Stephen Jahl <stephenjahl@gmail.com>
Merge pull request #2484 from sjahl/master

doc: Added bucket management commands to ops/crush-map

Reviewed-by: Loic Dachary <loic-201408@dachary.org>