doc: create add/remove metadata server

A new doc is created based on https://github.com/ceph/ceph/pull/22035#pullrequestreview-121600153.

Fixes: http://tracker.ceph.com/issues/24093
Signed-off-by: Jos Collin <jcollin@redhat.com>

diff --git a/doc/cephfs/add-remove-mds.rst b/doc/cephfs/add-remove-mds.rst
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+============================
+ Add/Remove Metadata Server
+============================
+
+.. important:: You must deploy at least one metadata server to use CephFS.
+    There is experimental support for running multiple metadata servers.
+    Do not run multiple active metadata servers in production.
+
+See `MDS Config Reference`_ for details on configuring metadata servers.
+
+
+Add a Metadata Server
+=====================
+
+#. Create an mds data point ``/var/lib/ceph/mds/mds.<your-mds-id>``.
+
+#. Edit ``ceph.conf`` and add MDS section. ::
+
+       [mds.<your-mds-id>]
+       host = {hostname}
+
+#. Create the authentication key, if you use CephX. ::
+
+       $ sudo ceph auth get-or-create mds.<your-mds-id> mon 'profile mds' mgr 'profile mds' mds 'allow *' osd 'allow *' > /var/lib/ceph/mds/ceph-<your-mds-id>/keying
+
+#. Start the service. ::
+
+       $ sudo service ceph start mds.<your-mds-id>
+
+#. The status of the cluster shows: ::
+
+       mds: cephfs_a-1/1/1 up  {0=c=up:active}, 3 up:standby
+
+Remove a Metadata Server
+========================
+
+.. note:: Ensure that if you remove a metadata server, the remaining metadata
+   servers will be able to service requests from CephFS clients. If that is not
+   possible, consider adding a metadata server before destroying the metadata
+   server you would like to take offline.
+
+If you have a metadata server in your cluster that you'd like to remove, you may use
+the following method.
+
+#. Create a new Metadata Server as shown in the above section.
+
+#. Stop the old Metadata Server and start using the new one. ::
+
+       $ ceph mds fail <mds name>
+
+#. Remove the ``/var/lib/ceph/mds/mds.<your-mds-id>`` directory on the old Metadata server.
+
+.. _MDS Config Reference: ../mds-config-ref