Update issue templates

Author: dpvc

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,16 @@
+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+Please submit MathJax bug reports to
+
+https://github.com/mathjax/MathJax/issues
+
+unless they are about structural issues in this repository (e.g., the package.json file, the compilation process, the repository layout, etc).  Bugs with how MathJax works should be reported at the link above.
+
+If you are not part of the MathJax team, it is unlikely that you should be posting an issue here.  Use the tracker linked above.