Documentation: cgroup: add documentation for net_cls cgroups.

This patch adds a new file, Documentation/cgroups/net_cls.txt, with info
about net_cls cgroups, and updates the 00-INDEX accordingly.

Signed-off-by: Rami Rosen <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/cgroups/00-INDEX

···
 	- Memory Resource Controller; implementation details.
 memory.txt
 	- Memory Resource Controller; design, accounting, interface, testing.
+net_cls.txt
+	- Network classifier cgroups details and usages.
 net_prio.txt
 	- Network priority cgroups details and usages.
 resource_counter.txt

Documentation/cgroups/net_cls.txt

···
+Network classifier cgroup
+-------------------------
+
+The Network classifier cgroup provides an interface to
+tag network packets with a class identifier (classid).
+
+The Traffic Controller (tc) can be used to assign
+different priorities to packets from different cgroups.
+
+Creating a net_cls cgroups instance creates a net_cls.classid file.
+This net_cls.classid value is initialized to 0.
+
+You can write hexadecimal values to net_cls.classid; the format for these
+values is 0xAAAABBBB; AAAA is the major handle number and BBBB
+is the minor handle number.
+Reading net_cls.classid yields a decimal result.
+
+Example:
+mkdir /sys/fs/cgroup/net_cls
+mount -t cgroup -onet_cls net_cls /sys/fs/cgroup/net_cls
+mkdir /sys/fs/cgroup/net_cls/0
+echo 0x100001 >  /sys/fs/cgroup/net_cls/0/net_cls.classid
+	- setting a 10:1 handle.
+
+cat /sys/fs/cgroup/net_cls/0/net_cls.classid
+1048577
+
+configuring tc:
+tc qdisc add dev eth0 root handle 10: htb
+
+tc class add dev eth0 parent 10: classid 10:1 htb rate 40mbit
+ - creating traffic class 10:1
+
+tc filter add dev eth0 parent 10: protocol ip prio 10 handle 1: cgroup