doc: Added transition from mkcephfs to ceph-deploy page.

fixes: #4756

Signed-off-by: John Wilkins <john.wilkins@inktank.com>

diff --git a/doc/rados/deployment/ceph-deploy-transition.rst b/doc/rados/deployment/ceph-deploy-transition.rst
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+==============================
+ Transitioning to ceph-deploy
+==============================
+
+If you have an existing cluster that you deployed with ``mkcephfs``, 
+you will need to make a few changes to your configuration to 
+ensure that your cluster will work with ``ceph-deploy``.
+
+Monitor Keyring
+===============
+
+You will need to add ``caps mon = "allow *"`` to your monitor keyring if it is
+not already in the keyring. By default, the monitor keyring is located under
+``/var/lib/ceph/mon/ceph-$id/keyring``. When you have added the ``caps``
+setting, your monitor keyring should look something like this::
+
+       [mon.]
+               key = AQBJIHhRuHCwDRAAZjBTSJcIBIoGpdOR9ToiyQ==
+               caps mon = "allow *" 
+               
+Adding ``caps mon = "allow *"`` will ease the transition from ``mkcephfs`` to
+``ceph-deploy`` by allowing ``ceph-create-keys`` to use the ``mon.`` keyring
+file in ``$mon_data`` and get the caps it needs.
+
+
+Use Default Paths
+=================
+
+Under the ``/var/lib/ceph`` directory, the ``mon`` and ``osd`` directories need
+to use the default paths.
+
+- **OSDs**: The path should be ``/var/lib/ceph/osd/ceph-$id``
+- **MON**: The path should be  ``/var/lib/ceph/mon/ceph-$id``
+
+Under those directories, the keyring should be in a file named ``keyring``.
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