Explain auto-close policy in ISSUE_TEMPLATE.md (exercism#1084)

* Explain auto-close policy in ISSUE_TEMPLATE.md

Per discussion in this forum thread: http://forum.exercism.org/t/update-github-issue-template-to-warn-users-new-issues-are-being-auto-closed/3676/2

* Ask people to use the forum instead in .github/ISSUE_TEMPLATE.md

Co-authored-by: Jeremy Walker <jez.walker@gmail.com>

---------

Co-authored-by: Jeremy Walker <jez.walker@gmail.com>

.github/ISSUE_TEMPLATE.md

@@ -1,7 +1,4 @@
 <!--
-If you're having trouble, please open an issue in
-http://github.com/exercism/exercism/issues.
+Hello. Please do NOT open issues in this repository. Instead use the Exercism Community Forum: https://forum.exercism.org
 
-If necessary, we will open actionable issues here
-about what needs to change.
 -->