Add an issue template to direct where things are posted

docs/issue_template.md

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+<!--
+Welcome!
+
+Help us stay on top of things (and get faster replies) by submitting them to
+the right place:
+
+* for support questions please use https://discourse.jupyter.org/c/binder
+* repo2docker development related issues https://github.com/jupyter/repo2docker/issues/new
+* if the https://mybinder.org site is down or has inaccessible pages: https://github.com/jupyterhub/mybinder.org-deploy/issues/new
+
+If you aren't sure where to go use https://discourse.jupyter.org/c/binder.
+
+-->