Adds issue template (thanks Duncan Betts)

.github/ISSUE_TEMPLATE

@@ -0,0 +1,12 @@
+## Checklist
+
+- [ ] I have included the output of ``celery -A proj report`` in the issue.
+      (if you are not able to do this, then at least specify the Celery
+       version affected).
+- [ ] I have verified that the issue exists against the `master` branch of Celery.
+
+## Steps to reproduce
+
+## Expected behavior
+
+## Actual behavior