Create issue template explaining deprecation

.github/issue_template.md

+## Deprecation notice
+
+Paperclip is currently undergoing
+[deprecation in favor of ActiveStorage](https://github.com/thoughtbot/paperclip/blob/master/MIGRATING.md).
+Maintainers of this repository will no longer be tending to new issues. We're leaving the issues page
+open so Paperclip users can still see & search through old issues, and continue existing discussions
+if they wish.