volume: Document multipath configuration

Add some lines about how to enable multipath for block storage.
A new README was added, because multipath is relevant for at least
FC and iSCSI.

Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>

pkg/volume/README.md

@@ -0,0 +1,27 @@
+## Multipath
+
+To leverage multiple paths for block storage, it is important to perform the
+multipath configuration on the host.
+If your distribution does not provide `/etc/multipath.conf`, then you can
+either use the following minimalistic one:
+
+    defaults {
+        find_multipaths yes
+        user_friendly_names yes
+    }
+
+or create a new one by running:
+
+    $ mpathconf --enable
+
+Finally you'll need to ensure to start or reload and enable multipath:
+
+    $ systemctl enable multipathd.service
+    $ systemctl restart multipathd.service
+
+**Note:** Any change to `multipath.conf` or enabling multipath can lead to
+inaccessible block devices, because they'll be claimed by multipath and
+exposed as a device in /dev/mapper/*.
+
+Some additional informations about multipath can be found in the
+[iSCSI documentation](iscsi/README.md)