.github: update enhancement issue template to point to KEPs

Author: nikhita

.github/ISSUE_TEMPLATE/enhancement.md

@@ -1,10 +1,16 @@
 ---
-name: Enhancement Request
-about: Suggest an enhancement to the Kubernetes project
+name: Enhancement Tracking Issue
+about: Provide supporting details for a feature in development
 labels: kind/feature
 
 ---
-<!-- Please only use this template for submitting enhancement requests -->
+<!-- Feature requests are unlikely to make progress as an issue.
+
+Instead, please suggest enhancements by engaging with SIGs on slack and mailing lists.
+
+A proposal that works through the design along with the implications of the change can be opened as a KEP:
+https://git.k8s.io/enhancements/keps#kubernetes-enhancement-proposals-keps
+-->
 
 #### What would you like to be added: