Document RCU and unloadable modules

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/RCU/00-INDEX

@@ -12,6 +12,8 @@ rcuref.txt
 	- Reference-count design for elements of lists/arrays protected by RCU
 rcu.txt
 	- RCU Concepts
+rcubarrier.txt
+	- Unloading modules that use RCU callbacks
 RTFP.txt
 	- List of RCU papers (bibliography) going back to 1980.
 torture.txt