diff --git a/Readme.md b/Readme.md
index 331abc6..b2db9e9 100644
--- a/Readme.md
+++ b/Readme.md
@@ -12,4 +12,6 @@ and 1e-6 and stored in according .csv files.
 You can find the three benchmarks in the `eval` directory. There is also a
 header file, which describes the benchmarks with its grid shape etc. 
 
-
+To write the resulting fields to a file, you can set the `BENCH_OUTPUT`
+variable, e.g. `BENCH_OUTPUT=1 ./bench`. The resulting fields are written to
+according .csv files.