Update README to add new file to the documentation.

Author: patr1ck-m

README.md

@@ -280,8 +280,9 @@ Tests use the `test` module from Effekt's standard library and return exit code
 │   └── test/                 # Test modules and data
 │       ├── aggregation_test.effekt
 │       ├── anomaly_detection_test.effekt
-│       ├── stream_input_test.effekt
+│       ├── counter_test.effekt
 │       ├── lib.effekt
+│       ├── stream_input_test.effekt
 │       ├── testdata.csv
 │       └── testdata_with_timestamp.csv
 ├── plot_data.py               # Visualization script for results