Add pull request template with CLA details (#21)

.github/pull_request_template.md

@@ -0,0 +1,11 @@
+<!--
+
+Before submitting a pull request:
+
+Contributions to this project must be accompanied by a Contributor License Agreement. You (or your employer) retain the copyright to your contribution; this simply gives us permission to use and redistribute your contributions as part of the project.
+
+Head over to https://cla.developers.google.com/ to see your current agreements on file or to sign a new one.
+
+You generally only need to submit a CLA once, so if you've already submitted one (even if it was for a different project), you probably don't need to do it again.
+
+-->