.github: add link to repo donation rules in issue template
While this link is also mentioned in the "Approvals" section, most
folks don't read it. So this commit explicitly mentions it in a comment
in the issue template.
@@ -8,6 +8,7 @@ assignees: ''
---
### New Repo, Staging Repo, or migrate existing
+<!-- Rules for migrating existing repos - https://git.k8s.io/community/github-management/kubernetes-repositories.md#rules-for-donated-repositories -->
e.g. new repository
### Requested name for new repository
