Create pull request template

* Referenced from project `rg3/youtube-dl` with necessary modifications

.github/PULL_REQUEST_TEMPLATE.md

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+## Please follow the guide below
+
+- You will be asked some questions, please read them **carefully** and answer honestly
+- Put an `x` into all the boxes [ ] relevant to your *pull request* (like that [x])
+- Use *Preview* tab to see how your *pull request* will actually look like
+
+- [ ] [Searched](https://github.com/shadowsocks/shadowsocks-windows/search?q=is%3Apr&type=Issues) for similar pull requests
+- [ ] Compiled the code with Visual Studio
+
+### What is the purpose of your *pull request*?
+- [ ] Bug fix
+- [ ] Improvement
+- [ ] New feature
+
+---
+
+### Description of your *pull request* and other information
+
+Explanation of your *pull request* in arbitrary form goes here. Please make sure the description explains the purpose and effect of your *pull request* and is worded well enough to be understood. Provide as much context and examples as possible.