documentation of some IP/ICMP snmp counters

The snmp_counter.rst explains the meanings of snmp counters. It also
provides a set of experiments (only 1 for this initial patch),
combines the experiments' resutls and the snmp counters'
meanings. This is an initial path, only explains a part of IP/ICMP
counters and provide a simple ping test.

Signed-off-by: yupeng <yupeng0921@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/Documentation/networking/index.rst b/Documentation/networking/index.rst
index bd89dae..6a47629 100644
--- a/Documentation/networking/index.rst
+++ b/Documentation/networking/index.rst
@@ -31,6 +31,7 @@
    net_failover
    alias
    bridge
+   snmp_counter
 
 .. only::  subproject