Create ISSUE_TEMPLATE.md (#238)

Credit goes to the people behind the phoenix issue template
Original: https://github.com/phoenixframework/phoenix/blob/master/ISSUE_TEMPLATE.md

ISSUE_TEMPLATE.md

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+<!--
+### Precheck
+
+* Before submitting an issue, please do a quick search and make sure the bug has not yet been reported
+* Ensure that this issue is related to the Arc library and not one of the dependencies listed in mix.exs (Ecto, Plug, etc.)
+
+-->
+
+### Environment
+
+* Elixir version (elixir -v):
+* Arc version (mix deps):
+* Arc dependencies when applicable (mix deps):
+* Operating system:
+
+### Expected behavior
+
+
+### Actual behavior