set pull request template

Author: grohong

PULL_REQUEST_TEMPLATE.md

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+## Types of changes
+<!--- What types of changes does your code introduce? Put an `x` in all the boxes that apply: -->
+- [ ] Bug fix (non-breaking change which fixes an issue)
+- [ ] New feature (non-breaking change which adds functionality)
+- [ ] Breaking change (fix or feature that would cause existing functionality to change)
+- [ ] I have read the **CONTRIBUTING** document.
+- [ ] My code follows the code style of this project.
+- [ ] My change requires a change to the documentation.
+- [ ] I have updated the documentation accordingly.
+- [ ] I have added tests to cover my changes.
+- [ ] All new and existing tests passed.