Add a contact link for Kotlinlang Slack (#3565)

Author: dkhalanskyjb

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,5 @@
+blank_issues_enabled: true
+contact_links:
+  - name: Kotlinlang Slack
+    url: https://surveys.jetbrains.com/s3/kotlin-slack-sign-up
+    about: Please ask and answer usage-related questions here.