misc: add docstring to bench_spans.py telling how to run it

ref #1182

diff --git a/misc/simulators/bench_spans.py b/misc/simulators/bench_spans.py
index 1855673f58d015cb333aa79b73b09da9062b01e7..e1e6e7ebd465153c91cc6121015e305d8d977df8 100644 (file)
--- a/misc/simulators/bench_spans.py
+++ b/misc/simulators/bench_spans.py
@@ -1,3 +1,13 @@
+"""
+To use this, get a trace file such as this one:
+
+wget http://tahoe-lafs.org/trac/tahoe-lafs/raw-attachment/ticket/1170/run-112-above28-flog-dump-sh8-on-nsziz.txt
+
+And run this command passing that trace file's name:
+
+python bench_spans.py run-112-above28-flog-dump-sh8-on-nsziz.txt
+"""
+
 from pyutil import benchutil
 
 from allmydata.util.spans import DataSpans