Specify the rules for release candidate feedback (#3990)

.github/ISSUE_TEMPLATE/rc_feedback.md

@@ -0,0 +1,28 @@
+---
+name: Release Candidate release feedback
+about: Something used to work, but broke in an RC release?
+title: 'RC:'
+labels: bug
+assignees: ''
+
+---
+
+<!--
+Thank you for participating in the Release Candidate program, and even better,
+finding a problem!
+
+Please double-check that this problem is not present in the latest stable
+release.
+-->
+
+**What broke?**
+
+Even a vague description suffices. If you want to search for a reproducer or
+narrow the issue down, this would help a lot, but the most important thing is
+letting us know that there is a problem at all as soon as possible. Otherwise,
+we'll just publish a stable release with this problem while you're looking for a
+reproducer!
+
+**Did I check that setting the version to the latest stable release fixes the problem?**
+
+Yes.