MNT: Update ISSUE_TEMPLATE.md with more explicit guidelines

Author: kernc

.github/issue_template.md

@@ -3,9 +3,14 @@
 
 ### Actual Behavior
 
+<!-- In case of a bug, attach full exception traceback. -->
+
 
 ### Steps to Reproduce
 
+<!-- In case of a bug, attach steps and code sample
+     with which the bug can be reproduced. -->
+
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