Document existence of non-production "fake" driver

Ironic has fake drivers for development use. Document that they are
not suitable for production.

Story: 1326269
Task: 9877
Change-Id: Ibe6d43e1740a95b1cb3886394afaf8545de00e54

doc/source/admin/drivers.rst

@@ -26,6 +26,7 @@ Hardware Types
   drivers/redfish
   drivers/snmp
   drivers/xclarity
+  drivers/fake
 
 Changing Hardware Types and Interfaces
 --------------------------------------

doc/source/admin/drivers/fake.rst

@@ -0,0 +1,36 @@
+===========
+Fake driver
+===========
+
+Overview
+========
+
+The ``fake-hardware`` hardware type is what it claims to be: fake. Use of this
+type or the ``fake`` interfaces should be temporary or limited to
+non-production environments, as the ``fake`` interfaces do not perform any of
+the actions typically expected.
+
+The ``fake`` interfaces can be configured to be combined with any of the
+"real" hardware interfaces, allowing you to effectively disable one or more
+hardware interfaces for testing by simply setting that interface to
+``fake``.
+
+Use cases
+=========
+
+Development
+-----------
+Developers can use ``fake-hardware`` hardware-type to mock out nodes for
+testing without those nodes needing to exist with physical or virtual hardware.
+
+Adoption
+--------
+Some OpenStack deployers have used ``fake`` interfaces in Ironic to allow an
+adoption-style workflow with Nova. By setting a node's hardware interfaces to
+``fake``, it's possible to deploy to that node with Nova without causing any
+actual changes to the hardware or an OS already deployed on it.
+
+This is generally an unsupported use case, but it is possible. For more
+information, see the relevant `post from CERN TechBlog`_.
+
+.. _`post from CERN TechBlog`: https://techblog.web.cern.ch/techblog/post/ironic-nova-adoption/