Add a PR template (Khan#171)

This is just what's already in the contributing docs, and is just to help both reviewers and authors remember to add changelog entries, check for tests, etc.

.github/pull_request_template.md

@@ -0,0 +1,15 @@
+<!--
+Thanks for your contribution! Check out the
+[contributing docs](https://github.com/Khan/genqlient/blob/main/docs/CONTRIBUTING.md)
+for more on contributing to genqlient.
+-->
+
+
+
+I have:
+- [ ] Written a clear PR title and description (above)
+- [ ] Signed the [Khan Academy CLA](https://www.khanacademy.org/r/cla)
+- [ ] Added tests covering my changes, if applicable
+- [ ] Included a link to the issue fixed, if applicable
+- [ ] Included documentation, for new features
+- [ ] Added an entry to the changelog