Add Issue template to model AASTeX issues

[augustfly]

Add an issue template https://help.github.com/articles/creating-an-issue-template-for-your-repository/

to help ensure that we collect minimum working examples from authors for bugs/problems OR properly direct style requests into ongoing discussion issues.

review the yt example (thanks @ngoldbaum ) https://raw.githubusercontent.com/yt-project/yt/master/.github/ISSUE_TEMPLATE.md