Update feature_request.yml

Author: DannyDainton

.github/ISSUE_TEMPLATE/feature_request.yml

@@ -3,6 +3,13 @@ description: Request a new feature that you would like to be part of the Postman
 title: "<title>"
 labels: "feature"
 body:
+- type: checkboxes
+  attributes:
+    label: Is there an existing request for this feature?
+    description: Please search to see if a feature request already exists. [Github Issue Tracker](https://github.com/postmanlabs/postman-app-support/issues)
+    options:
+    - label: I have searched the existing issues for this feature request
+      required: true
 - type: textarea
   attributes:
     label: Is your feature request related to a problem?