doc(ci): adding constraint on the branch name

Signed-off-by: Raphael Defosseux <raphael.defosseux@eurecom.fr>

CONTRIBUTING.md

@@ -12,6 +12,8 @@ We want to make contributing to this project as easy and transparent as possible
       * This decision was made for the license reasons.
       * The Continuous Integration will reject your merge request.
    - All merge requests SHALL have `develop` branch as target branch.
+   - All merge requests SHALL have source branch names that SHALL not contain the `/` character.
+      * We are using a `docker` registry scheme and the image names are based on the source branch name.
 
 ## Coding Styles ##