Add new requirements for stub in module guidelines

sites/docs/src/content/docs/guidelines/components/modules.md

@@ -324,6 +324,43 @@ See the [Bash manual on file operators](https://tldp.org/LDP/abs/html/fto.html)
 
 Alternate suggestions include using `grep -c` to search for a valid string match, or other tool which will appropriately error when the expected output is not successfully created.
 
+### Stubs
+
+#### Stub block must exist
+
+[A stub block](https://www.nextflow.io/docs/latest/process.html#stub) MUST exist for all modules. This is a block of code that replaces the `script` command when the option `-stub` is set. This enables quick testing of the workflow logic, as a "dry-run".
+
+#### Stub block prefix and versions
+
+The stub block MUST include the same variables (e.g. `prefix`) and HEREDOC code as the main script block. 
+
+#### Stub files for all output channels 
+
+The stub block MUST include the creation of at least one file for every output channel (both mandatory and optional), generated with touch, e.g.
+
+```groovy
+stub:
+"""
+touch ${prefix}.txt
+"""
+```
+
+Ideally, the stub block should reproduce as much as possible the number of, and filenames structure, of the files expected as output.
+
+#### Stub gzip files must use echo and pipe
+
+Stub files that should be output as gzip compressed, MUST use the syntax in the following example:
+
+```bash
+echo "" | gzip > ${prefix}.txt.gz
+```
+
+:::info{title="Rationale" collapse}
+Simply touching a file with the file name ending in `.gz` will break nf-test's Gzip file parser, as the file is not actually gzipped and thus cannot be read.
+
+Therefore we must make sure we generate a valid gzipped file for nf-test to accept it during tests.
+:::
+
 ## Naming conventions
 
 ### Name format of module files