Update feature_request.md

Author: fcollonval

.github/ISSUE_TEMPLATE/feature_request.md

@@ -6,6 +6,8 @@ labels: type:Enhancement
 assignees: ''
 
 ---
+<!--Please check the open issue to see if your feature is not already reported.
+If this is the case, +1 it to show your interest.-->
 
 ## Is your feature request related to a problem? Please describe.
 <!--A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]-->