Allow explicit setting of a number of files

We currently only allow filename=foo for one file, add the possibility
to specify any number of files by seperating with a colon.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/HOWTO b/HOWTO
index f9e3fc7..5df2607 100644
--- a/HOWTO
+++ b/HOWTO
@@ -205,7 +205,11 @@
 		files between threads in a job or several jobs, specify
 		a filename for each of them to override the default. If
 		the ioengine used is 'net', the filename is the host and
-		port to connect to in the format of =host:port.
+		port to connect to in the format of =host:port. If the
+		ioengine is file based, you can specify a number of files
+		by seperating the names with a ':' colon. So if you wanted
+		a job to open /dev/sda and /dev/sdb as the two working files,
+		you would use filename=/dev/sda:/dev/sdb
 
 rw=str		Type of io pattern. Accepted values are: