Document passings args to formatters (#4688)

* Document passings args to formatters

Author: jeddy3

docs/developer-guide/formatters.md

@@ -79,6 +79,20 @@ And the second argument (`returnValue`) is an object with one or more of the fol
 }
 ```
 
+## Passing arguments
+
+You can use environmental variables in your formatter. For example, pass `SKIP_WARNINGS`:
+
+```console
+SKIP_WARNINGS=true stylelint "*.css" --custom-formatter ./my-formatter.js
+```
+
+Alternatively, you can create a separate formatting program and pipe the output from the built-in JSON formatter into it:
+
+```console
+stylelint -f json "*.css" | my-program-that-reads-JSON --option
+```
+
 ## `stylelint.formatters`
 
 stylelint's internal formatters are exposed publicly in `stylelint.formatters`.