[INTERNAL] GitHub: Add PULL_REQUEST_TEMPLATE

Author: RandomByte

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,9 @@
+**Thank you for your contribution!** 🙌
+
+To get it merged faster, kindly review the checklist below:
+
+## Pull Request Checklist
+- [ ] Reviewed the [Contributing Guidelines](https://github.com/SAP/ui5-tooling/blob/master/CONTRIBUTING.md#-contributing-code)
+    + Especially the [How to Contribute](https://github.com/SAP/ui5-tooling/blob/master/CONTRIBUTING.md#how-to-contribute) section 
+- [ ] [No merge commits](https://github.com/SAP/ui5-tooling/blob/master/docs/Guidelines.md#no-merge-commits)
+- [ ] [Correct commit message style](https://github.com/SAP/ui5-tooling/blob/master/docs/Guidelines.md#commit-message-style)