[ProjectMaintenance]建立PR模板

[birdflyi]

一起来建立PR模板吧~
我们可以参考https://raw.githubusercontent.com/X-lab2017/OSSDevGov2021/main/.github/PULL_REQUEST_TEMPLATE.md ，做一个PR模板用于解决文献入库等常规Issue，示例：

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## Purpose

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-  Fix # <!--As well as Close # -->

## Changes

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- Your changes

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为什么推荐使用PR模板？（选读）
不推荐多次commit解决某一个Issue。出于美观，以及考虑到sha-1码查询困难（需要从commit中获取提交信息）、关联到Issue主题难度大、github不支持预览等方面考虑，推荐使用PR解决对应的Issue。示例：Issue #69 Fixed by PR #70 。
为了方便自动化关联，我们需要一个统一的PR模板。
使用PR的好处：在PR中我们可以提供标签、增加主题说明、多次提交、多人讨论、完整的指派和review流程、增加bus系数、自动化程度高、有github预览等好处~