add pull request template discouraging contributions

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,10 @@
+These style guides are copies of Google's internal style guides to
+assist developers working on Google owned and originated open source
+projects. Changes should be made to the internal style guide first and
+only then copied here.
+
+Unsolicited pull requests are not accepted and will be closed without
+comment.
+
+If this PR does anything other than update a style guide with changes
+already made to the internal copy, please do not create it.