xfrm: add documentation for xfrm device offload api

Add a writeup on how to use the XFRM device offload API, and
mention this new file in the index.

Signed-off-by: Shannon Nelson <shannon.nelson@oracle.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
diff --git a/Documentation/networking/00-INDEX b/Documentation/networking/00-INDEX
index 7a79b35..f5d642c 100644
--- a/Documentation/networking/00-INDEX
+++ b/Documentation/networking/00-INDEX
@@ -228,6 +228,8 @@
 	- general info on X.25 development.
 x25-iface.txt
 	- description of the X.25 Packet Layer to LAPB device interface.
+xfrm_device.txt
+	- description of XFRM offload API
 xfrm_proc.txt
 	- description of the statistics package for XFRM.
 xfrm_sync.txt