Create PULL_REQUEST_TEMPLATE.md

Author: Amin Mahboubi

PULL_REQUEST_TEMPLATE.md

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+# Submit a pull request
+
+## CLA
+
+- [ ] I have signed the [Stream CLA](https://docs.google.com/forms/d/e/1FAIpQLScFKsKkAJI7mhCr7K9rEIOpqIDThrWxuvxnwUq2XkHyG154vQ/viewform) (required).
+- [ ] The code changes follow best practices
+- [ ] Code changes are tested (add some information if not applicable)
+
+## Description of the pull request