Add support for HDFS IO engine

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/HOWTO b/HOWTO
index 87346ae..d728353 100644
--- a/HOWTO
+++ b/HOWTO
@@ -694,6 +694,8 @@
 				having to go through FUSE. This ioengine
 				defines engine specific options.
 
+			hdfs	Read and write through Hadoop (HDFS).
+
 			external Prefix to specify loading an external
 				IO engine object file. Append the engine
 				filename, eg ioengine=external:/tmp/foo.o