Document 'openfiles' option

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/HOWTO b/HOWTO
index ea943ee..d1a3ad1 100644
--- a/HOWTO
+++ b/HOWTO
@@ -253,6 +253,10 @@
 
 nrfiles=int	Number of files to use for this job. Defaults to 1.
 
+openfiles=int	Number of files to keep open at the same time. Defaults to
+		the same as nrfiles, can be set smaller to limit the number
+		simultaneous opens.
+
 file_service_type=str  Defines how fio decides which file from a job to
 		service next. The following types are defined: