Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -114,7 +114,7 @@ Notes:
   ```
 
 
-### Step 6: Fetch test data
+### Step 6: Create or fetch test data
 
 To help develop the component, it's interesting to have some test data available. In most cases, we can use the test data from the Snakemake wrappers. 
 
@@ -133,6 +133,8 @@ cp -r /tmp/snakemake-wrappers/bio/xxx/test/* src/xxx/test_data
 EOF
 ```
 
+The test data should be suitable for testing this component. Ensure that the test data is small enough: ideally <1KB, preferably <10KB, if need be <100KB.
+
 ### Step 7: Add arguments for the input files
 
 By looking at the help file, we add the input arguments to the config file. Here is an example of the input arguments of an existing component.