Add debugging info

Equivalent info should be here: https://github.com/nf-core/website/blob/main/src/content/docs/contributing/contributing_to_pipelines.md

nf_core/pipeline-template/.github/CONTRIBUTING.md

@@ -31,6 +31,9 @@ If you're not used to this workflow with git, you can start with some [docs from
 
 ## Tests
 
+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.