Add pull request template with checklist

Signed-off-by: Vincent Petry <vincent@nextcloud.com>

.github/pull_request_template.md

@@ -0,0 +1,18 @@
+
+* Resolves: # <!-- related github issue -->
+
+## Summary
+
+
+## TODO
+
+- [ ] ...
+
+## Checklist
+
+- Code is [properly formatted](https://docs.nextcloud.com/server/latest/developer_manual/digging_deeper/continuous_integration.html#linting)
+- [Sign-off message](https://github.com/src-d/guide/blob/master/developer-community/fix-DCO.md) is added to all commits
+- [ ] Tests ([unit](https://docs.nextcloud.com/server/latest/developer_manual/app_development/tutorial.html#unit-tests), [integration](https://docs.nextcloud.com/server/latest/developer_manual/app_development/tutorial.html#integration-tests), api and/or acceptance) are included
+- [ ] Screenshots before/after for front-end changes
+- [ ] Documentation ([manuals](https://github.com/nextcloud/documentation/) or wiki) has been updated or is not required
+- [ ] [Backports requested](https://github.com/nextcloud/backportbot/#usage) where applicable (ex: critical bugfixes)