doc: add page for Rook module

This is pretty sparse because users don't really
interact with the module, they interact with other
modules that use it.

Signed-off-by: John Spray <john.spray@redhat.com>

doc/dev/kubernetes.rst

@@ -1,4 +1,6 @@
 
+.. _kubernetes-dev:
+
 =======================================
 Hacking on Ceph in Kubernetes with Rook
 =======================================

doc/mgr/index.rst

@@ -40,3 +40,4 @@ sensible.
     Iostat plugin <iostat>
     Crash plugin <crash>
     Devicehealth plugin <devicehealth>
+    Rook plugin <rook>

doc/mgr/rook.rst

@@ -0,0 +1,36 @@
+
+=============================
+Rook orchestrator integration
+=============================
+
+Rook (https://rook.io/) is an orchestration tool that can run Ceph inside
+a Kubernetes cluster.
+
+The ``rook`` module provides integration between Ceph's orchestrator framework
+(used by modules such as ``dashboard`` to control cluster services) and
+Rook.
+
+Orchestrator modules only provide services to other modules, which in turn
+provide user interfaces.  To try out the rook module, you might like
+to use the :ref:`Orchestrator CLI <orchestrator-cli-module>` module.
+
+Requirements
+------------
+
+- Running ceph-mon and ceph-mgr services that were set up with Rook in
+  Kubernetes.
+- A sufficiently recent version of Rook. (**TODO: update once required Rook
+  tweaks are in a release**)
+
+If you are a developer, please see :ref:`kubernetes-dev` for instructions
+on setting up a development environment to work with this.
+
+Configuration
+-------------
+
+Because a Rook cluster's ceph-mgr daemon is running as a Kubernetes pod, 
+the ``rook`` module can connect to the Kubernetes API without any explicit
+configuration.
+
+
+