Add instructions for external contributors

CONTRIBUTING.org

@@ -83,9 +83,10 @@ External users need to first [[https://gitlab-account.inria.fr/][register]] and
 tab of the [[https://gitlab.inria.fr/users/sign_in][sign_in]] page.
 *** Post an issue
 Create a new issue (see [[https://gitlab.inria.fr/solverstack/chameleon/issues][issues]]) presenting your contribution proposal (feature,
-fix, ...). The Chameleon team will set up a contribution branch for you on which
-you will be able to push, and will provide you further instructions to share
-your contributions and perform your merge request.
+fix, ...). The Chameleon team will set up a contribution branch for you. You can
+attach a patch to the issue, which we will use in this case to initiate the
+branch. In any case, we will then provide you with further instructions to work
+on the branch and eventually perform your merge request.
 
 * Configure a runner to test your branch
   To be effectively merged, your branch must be tested through the