Document the use of the USR1 signal for dumping runtime statistics

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/fio.1 b/fio.1
index bcae37b..969c4a1 100644
--- a/fio.1
+++ b/fio.1
@@ -1203,6 +1203,10 @@
 Disk utilization.
 .RE
 .PD
+.P
+It is also possible to get fio to dump the current output while it is
+running, without terminating the job. To do that, send fio the \fBUSR1\fR
+signal.
 .SH TERSE OUTPUT
 If the \fB\-\-minimal\fR option is given, the results will be printed in a
 semicolon-delimited format suitable for scripted use - a job description