xdp: referece XDP router from APNIC

docs/_include/interface-xdp.txt

@@ -17,6 +17,9 @@
     this interface, as XDP is handled way earlier in the driver then iptables/
     nftables.
 
+  Enabling this feature will only load the XDP router code as described here:
+  https://blog.apnic.net/2020/04/30/how-to-build-an-xdp-based-bgp-peering-router/
+
   Example:
 
   .. code-block:: none