add issue template

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,11 @@
+This repository publishes copies of Google's internal style guides to
+assist developers working on Google owned and originated open source
+projects. Development on these guides does not take place here.
+
+Substantive changes to the style rules and suggested new rules should
+not be submitted as issues in this repository. Material changes must be
+proposed, discussed, and approved on the internal forums first.
+
+If an issue points out a simple mistake — a typo, a broken link, etc. —
+then a correction might be made. However there is no commitment to do
+so. Issues are normally closed without comment.