Add issue config file with link to grpc/grpc (#937)

Author: JamesNK

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,5 @@
+blank_issues_enabled: true
+contact_links:
+  - name: Issue with Grpc.Core
+    url:  https://github.com/grpc/grpc/issues/new/choose
+    about: Please open issues relating to Grpc.Core in grpc/grpc.