Create PULL_REQUEST_TEMPLATE.md

.github/PULL_REQUEST_TEMPLATE.md

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+Description：简要描述这次PR的内容
+
+Main reason: 做出这次修改的原因
+
+Checklist  检查下面各项是否完成
+
+Please feel free to remove inapplicable items for your PR.
+
+-	[ ] The PR title starts with [$CATEGORY] (such as [Models], [Modules], [Core], [io], [Doc], 分别对应各个子模块)
+-	[ ] Changes are complete (i.e. I finished coding on this PR)  代码写完了
+-	[ ] All changes have test coverage  修改的地方经过测试。对于可复用部分的修改，例如core/和modules/，测试代码必须提供。其他部分建议提供。
+-	[ ] Code is well-documented  注释写好，文档会从注释中自动抽取
+-	[ ] To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change  这种情况请找核心开发人员
+
+Changes: 逐项描述修改的内容
+-	Switch to sparse_coo_matrix for torch v1.0. #282
+- Fix bug that nx graph to dgl graph is not properly converted. #286