documentation: Add reference to 2014 RCU API LWN article

Reported-by: Jim Roskind <jar@roskind.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Documentation/RCU/whatisRCU.txt

@@ -5,6 +5,7 @@ to start learning about RCU:
 2.	What is RCU? Part 2: Usage   http://lwn.net/Articles/263130/
 3.	RCU part 3: the RCU API      http://lwn.net/Articles/264090/
 4.	The RCU API, 2010 Edition    http://lwn.net/Articles/418853/
+5.	The RCU API, 2014 Edition    http://lwn.net/Articles/609904/
 
 
 What is RCU?