chore: Add Pull Request Checklist (#922)

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,9 @@
+<!--
+Thanks for your contribution!
+Please check the following to make sure your contribution follows our guideline:
+-->
+
+## Pull Request Checklist
+
+- [ ] My code is covered by tests
+- [ ] I will [merge the PR as breaking change](https://github.com/wireapp/wire-web-packages/wiki/Releases#create-major-release), if the API contract changes