Add PR template

Author: takluyver

.github/pull_request_template.md

@@ -0,0 +1,8 @@
+If you're adding a project to the Python 3 statement, please ensure you have:
+
+- [ ] Discussed dropping Python 2 support with all maintainers
+- [ ] Discussed signing the statement with all maintainers
+- [ ] Added a suitable logo if the project has one
+  - [ ] Approximately square logos generally fit better than banners
+  - [ ] Compressed the logo to save bandwidth (run `scripts/squash-images.sh` on it)
+- [ ] Checked the Netlify preview made from the pull request