Add hint about debug profile

src/content/docs/contributing/adding_pipelines.md

@@ -131,6 +131,9 @@ to make sure that your workflow passes all of the nf-core compatibility tests.
 The automated tests on Github Actions also run this, so you should get a
 notification from GitHub if something breaks.
 
+When testing the pipeline you can add the `debug` profile (`-profile debug`) to the Nextflow command line,
+to enable warnings about process selectors, show additional debug output and disable cleanup.
+
 ## Running with test data
 
 Whilst the linting tests are good, they're not sufficient by themselves.

src/content/docs/contributing/contributing_to_pipelines.md

@@ -48,6 +48,9 @@ If you're new to working with git, you can view the [GitHub pull requests docume
 
 ## Testing
 
+You can optionally test your changes by running the pipeline locally. Then it is recommended to use the `debug` profile to
+receive warnings about process selectors and other debug info. Example: `nextflow run . -profile debug,test,docker --outdir <OUTDIR>`.
+
 When you create a pull request with changes, [GitHub Actions](https://github.com/features/actions) will run automatic tests.
 Typically, pull-requests are only fully reviewed when these tests are passing, though of course, we can help out before then.