document command

src/arriba/test_data/README.md

@@ -5,4 +5,9 @@ Test data was obtained from https://github.com/snakemake/snakemake-wrappers/tree
 __author__ = "Jan Forster"
 __copyright__ = "Copyright 2019, Jan Forster"
 __email__ = "[email protected]"
-__license__ = "MIT"
+__license__ = "MIT"
+
+```bash
+git clone --depth 1 --single-branch --branch master https://github.com/snakemake/snakemake-wrappers /tmp/snakemake-wrappers
+cp -r /tmp/snakemake-wrappers/bio/arriba/test/* src/arriba/test_data
+```