Add discussion link to issue template

Before KEPs are created, they are intended to be discussed and
socialized with the owning SIG. This field adds a discoverability point
between the tracking issue and the original conversation around the KEP.

Author: mrbobbytables

ISSUE_TEMPLATE.md

@@ -1,6 +1,7 @@
 # Enhancement Description
 - One-line enhancement description (can be used as a release note):
 - Kubernetes Enhancement Proposal: <!-- link to kubernetes/enhancements file; if none yet, link to PR -->
+- Discussion Link: <!-- link to SIG mailing list thread, meeting, or recording where the Enhancement was discussed before KEP creation -->
 - Primary contact (assignee):
 - Responsible SIGs:
 - Enhancement target (which target equals to which milestone):