pull_request_template: new file

Author: ken-matsui

.github/pull_request_template.md

@@ -0,0 +1,8 @@
+<!--
+Complete the following checklist and remove it from the description.
+-->
+
+## Checklist
+
+- [ ] Did you follow [CONTRIBUTING.md](https://github.com/poac-dev/poac/blob/main/CONTRIBUTING.md)?
+- [ ] Did you follow [Pull Request Style](https://github.com/poac-dev/poac/blob/main/CONTRIBUTING.md#pull-request-style)?