update contributing guide

CONTRIBUTING.md

@@ -156,7 +156,8 @@ Implement the `parse_output` function.
 The edges in the Local Neighborhood output have the same format as the input, `<vertex1>|<vertex2>`.
 Convert these to be tab-separated vertex pairs followed by a tab and a `1` at the end of every line, which indicates all edges have the same rank.
 See the `add_rank_column` function in `src.util.py`.
-The output should have the format `<vertex1> <vertex2> 1`.
+Make sure header = True when the file is created.
+The output should have the format `<vertex1> <vertex2> 1 U`.
 
 ### Step 4: Make the Local Neighborhood wrapper accessible through SPRAS
 Import the new class `LocalNeighborhood` in `src/runner.py` so the wrapper functions can be accessed.