Add pull request template (#50)

Ref https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/creating-a-pull-request-template-for-your-repository

.github/pull_request_template.md

@@ -0,0 +1,10 @@
+<!-- Describe why/what/how etc. -->
+
+### Checklist
+
+Check all and take necessary actions when you open this pull request.
+(These help auto-generation for release notes)
+
+- [ ] Add the `enhancement` label if this adds a new feature.
+- [ ] Add the `bug` label if this fixes a bug.
+- [ ] Start the title with `[Breaking]` if this includes a breaking change.