Create PULL_REQUEST_TEMPLATE.md

.github/PULL_REQUEST_TEMPLATE.md

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+## Description
+<!-- Brief description. Refer to the related issues if existed.
+It'll be great if relevant reviewers can be assigned as well.-->
+
+## Checklist
+Please feel free to remove inapplicable items for your PR.
+- [ ] The PR title starts with [$CATEGORY] (such as [NN], [Model], [Doc], [Feature]])
+- [ ] I've leverage the `lint.sh` to beautify the python and c++ code.
+- [ ] The PR is complete and small, read the [Google eng practice (CL equals to PR)](https://google.github.io/eng-practices/review/developer/small-cls.html) to understand more about small PR. In DGL, we consider PRs with less than 200 lines of core code change are small (example, test and documentation could be exempted).
+- [ ] Code is well-documented.
+- [ ] Related issue is referred in this PR.
+
+## Changes
+<!-- You could use following template
+- [ ] Feature1, tests, (and when applicable, API doc)
+- [ ] Feature2, tests, (and when applicable, API doc)
+-->