Add issue template, resolves #1773

On issue #1773, we wanted use issue templates to accomplish two things:
1. guide questions into StackOverflow and IRC channel
2. not duplicate CONTRIBUTING content
To resolve these, ISSUE_TEMPLATE is added to remind users not
to ask questions.

.github/ISSUE_TEMPLATE.rst

@@ -0,0 +1,2 @@
+The issue tracker is a tool to address bugs.
+Please use `#pocoo` IRC channel on freenode or `StackOverflow` for questions.