Add Gitter badge

Author: gitter-badger

README.md

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 README
 ===========================
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+[![Join the chat at https://gitter.im/guodongxiaren/README](https://badges.gitter.im/guodongxiaren/README.svg)](https://gitter.im/guodongxiaren/README?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
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 该文件用来测试和展示书写README的各种markdown语法。GitHub的markdown语法在标准的markdown语法基础上做了扩充，称之为`GitHub Flavored Markdown`。简称`GFM`，GFM在GitHub上有广泛应用，除了README文件外，issues和wiki均支持markdown语法。
 
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