virtio: Add text copy of spec to Documentation/virtual.

As suggested by Christoph Hellwig.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
diff --git a/Documentation/virtual/00-INDEX b/Documentation/virtual/00-INDEX
index fe0251c..8e60199 100644
--- a/Documentation/virtual/00-INDEX
+++ b/Documentation/virtual/00-INDEX
@@ -8,3 +8,6 @@
 	- Extremely simple hypervisor for experimental/educational use.
 uml/
 	- User Mode Linux, builds/runs Linux kernel as a userspace program.
+virtio.txt
+	- Text version of draft virtio spec.
+          See http://ozlabs.org/~rusty/virtio-spec