Add Issue template.

Author: marc0der

ISSUE_TEMPLATE.md

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+We require a conversation to take place in the appropriate Gitter Room before raising a new Github Issue. Please note that **issues will be closed immediately if prior discussion did not take place**. We need to do this to help manage the quality and validity of Issues raised on this project.
+
+Please tick one:
+- [ ] this is a new Feature Request, a conversation was started in the [Gitter CLI Development Room](https://gitter.im/sdkman/cli-dev).
+- [ ] this is a new Bug Report or Issue, a conversation was started in [Gitter User Issues Room](https://gitter.im/sdkman/user-issues).
+
+Please explain the Issue / Feature Request here: