Added issue template so hopefully issues are easier to understand and…

… reproduce

ISSUE_TEMPLATE.md

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+# Submitting bugs
+
+Make sure to review these points before submitting issues - thank you!
+
+- **Include reproducible code**: we should be able to paste it into an editor, compile and run it and get the same error as you. Otherwise it's impossible for us to reproduce the bug.
+- Don't **only** use `play.crystal-lang.org` or `carc.in`: code might be lost and the issue will remain incomplete. Write code in the issue itself.
+- Reduce code, if possible, to the minimum size that reproduces the bug.
+- If all of the above is impossible due to a large project, create a branch that reproduces the bug and point us to it.
+- Include Crystal compiler version (`crystal -v`) and OS. If possible, try to see if the bug still reproduces on master.
+
+# Submitting questions
+
+- If the question is something like "how do I do X?", consider using [StackOverflow](http://stackoverflow.com/questions/tagged/crystal-lang)
+- If the question is about a shard, consider asking the question in that shard