Add a bit of usage documentation for the storage space

Change-Id: I3b6a63a4ad63fc404f92a0d230b0ad5157529935

src/README.md

@@ -144,6 +144,41 @@ and then (with juju 2.x):
 Note that this OpenStack system will need to be configured (in terms of
 networking, images, etc.) before testing can commence.
 
+## Spaces
+
+This charm can optionally dedicate a provider's physical
+network to serving Ganesha NFS shares. It does so through its
+support for Juju spaces.
+
+The charm uses a space called `tenant-storage` and it should be accessible
+(routed is ok) to all tenants that expect to access the Manila shares. The
+easiest way to ensure this access is to create a provider network in OpenStack
+that is mapped to the same network layer as this space is. For example, the
+storage space is mapped to VLAN 120, then an OpenStack administrator should
+create a provider network that maps to the same VLAN. For example:
+
+    openstack network create \
+        --provider-network-type vlan \
+        --provider-segment 120 \
+        --share \
+        --provider-physical-network physnet1 \
+        tenant-storage
+    openstack subnet create tenant \
+        --network=tenant-storage \
+        --subnet-range 10.1.10.0/22 \
+        --gateway 10.1.10.1 \
+        --allocation-pool start=10.1.10.50,end=10.1.13.254
+
+When creating the space in MAAS that corresponds to this network, be sure that
+DHCP is disabled in this space. If MAAS performs any additional allocations in
+this space, ensure that the range configured for the subnet in Neutron does not
+overlap with the MAAS subnets.
+
+If dedicating a network space is not desired, it is also possible to use
+Ganesha over a routed network. Manila's IP access restrictions will still be
+used to secure access to Ganesha even if the network is not a Neutron managed
+network.
+
 # Bugs
 
 Please report bugs on [Launchpad](https://bugs.launchpad.net/charm-manila-ganesha/+filebug).