add info about UX label to feature proposal to CONTRIBUTING.md

CONTRIBUTING.md

@@ -286,7 +286,10 @@ might be edited to make them small and simple.
 
 Please submit Feature Proposals using the ['Feature Proposal' issue template](.gitlab/issue_templates/Feature Proposal.md) provided on the issue tracker.
 
-For changes in the interface, it can be helpful to create a mockup first.
+For changes in the interface, it is helpful to include a mockup. Issues that add to, or change, the interface should 
+be given the ~"UX" label. This will allow the UX team to provide input and guidance. You may 
+need to ask one of the [core team] members to add the label, if you do not have permissions to do it by yourself. 
+
 If you want to create something yourself, consider opening an issue first to
 discuss whether it is interesting to include this in GitLab.