Warn the user in redis.conf that VM is deprecated and discouraged

redis.conf

@@ -294,6 +294,9 @@ no-appendfsync-on-rewrite no
 
 ################################ VIRTUAL MEMORY ###############################
 
+### WARNING! Virtual Memory is deprecated in Redis 2.4
+### The use of Virtual Memory is strongly discouraged.
+
 # Virtual Memory allows Redis to work with datasets bigger than the actual
 # amount of RAM needed to hold the whole dataset in memory.
 # In order to do so very used keys are taken in memory while the other keys