Add 'null' verify option

For testing purposes.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/HOWTO b/HOWTO
index 8312d93..a5be9db 100644
--- a/HOWTO
+++ b/HOWTO
@@ -555,6 +555,10 @@
 			crc32	Use a crc32 sum of the data area and store
 				it in the header of each block.
 
+			null	Only pretend to verify. Useful for testing
+				internals with ioengine=null, not for much
+				else.
+
 		This option can be used for repeated burn-in tests of a
 		system to make sure that the written data is also
 		correctly read back.