Add a PR template; warn about transformer changes. (#500)

Author: twifkak

.github/pull_request_template.md

@@ -0,0 +1,8 @@
+<!--
+Changes to files in `transformers/`, `internal/url/`, or `cmd/transform/` need
+to be submitted in the Google repo first and then synced out to GitHub.
+Non-Google contributors are encouraged to send PRs, but reviewers will patch
+approved changes internally. Please read
+https://github.com/ampproject/amppackager/wiki/Contributing#working-on-transformers
+for details.
+-->