Create pull_request_template.md (AtsushiSakai#397)

Author: AtsushiSakai

.github/pull_request_template.md

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+<!-- 
+Thanks for contributing a pull request! 
+Please ensure that your PR satisfies the checklist before submitting:
+- [] This project only accept codes for python 3.8 or higher.
+- [] If you add a new algorithm sample code, please add a unit test file under `test` dir.
+     This sample test code might help you : https://github.com/AtsushiSakai/PythonRobotics/blob/master/tests/test_a_star.py
+- [] If you fix a bug of existing code please add a test function in the test code to show the issue was solved.
+- [] Please fix all issues on CI (All CI should be green), before code review.
+
+Note that this is my hobby project; I appreciate your
+patience during the review process.
+
+Again, thanks for contributing!
+-->
+
+#### Reference issue
+<!--Example: fix #392-->
+
+#### What does this implement/fix?
+<!--Please explain your changes.-->
+
+#### Additional information
+<!--Any additional information you think is important.-->