Create PULL_REQUEST_TEMPLATE.md

.github/PULL_REQUEST_TEMPLATE.md

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+<!--
+Describe the change in summary section, including rationale and degin decisions.
+Include "Fixes #nnn" if you are fixing an existing issue.
+
+If you have more information you want to add, write them in "Additional
+Information" section. This is usually used to help others understand your
+motivation behind this change. A step-by-step reproduction of the problem is
+helpful if there is no related issue.
+-->
+
+##### Summary
+
+##### Additional Information