Create issue_template.md

Give bug / feature request reporters a template to fill out.

.github/issue_template.md

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+## Expected or desired behavior
+
+Please describe the behavior you expected / want
+
+## Actual behavior
+
+Please describe the behavior you are actually seeing. 
+
+For bug-reports, if applicable, include error messages, emacs stack
+traces etc. Try to be as specific as possible - when do you see this
+happening? Does it happen always?  Sometimes? How often?
+
+## Steps to reproduce
+
+For bug-reports, please describe in as much details as possible how
+one can reproduce the problem*.
+
+## Versions of mu, mu4e/emacs, operating system etc.
+
+## Any other detail