add information on which branch to contribute to (#303)

Co-authored-by: Michael R. Crusoe <[email protected]>

Author: Fienne

CONTRIBUTING.md

@@ -43,7 +43,12 @@ and to meet some of our community members.
     you can submit a pull request (PR).
     Instructions for doing this are [included below][using-github].
 
-4. To build and run the user guide locally, see **Building** below
+4. To build and run the user guide locally, see **Building** below  
+
+**Note:** The published version of the user guide <https://www.commonwl.org/user_guide/> is built from the `release` branch.
+New changes are gathered on the default (`main`) branch which is built at <https://common-workflow-languageuser-guide.readthedocs.io/en/latest/>
+as a preview. Once we collect many changes from the `main` branch, we merge them into the `release` branch.
+
 
 Pull requests include an automatic preview provided by
 [ReadTheDocs](https://readthedocs.org/projects/common-workflow-languageuser-guide/).