Add CONTRIBUTING.md

address #40

CONTRIBUTING.md

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+# How to contribute
+
+## Ways to improve the documents
+
+* Submit a issue
+
+  If you find errors in documents or want to improve, this is the easiest way to help.
+  You can put your document in any format in the issue comments, then a core developer
+  will format it with reStructureText syntax and commit it into the repo.
+
+* Send a pull request
+
+  This way is for those who are familar with
+  [Git workflow](https://guides.github.com/introduction/flow/) and the
+  [reStructureText](http://www.sphinx-doc.org/en/stable/rest.html).
+  A PR needs to be reviewed by at least one person before merging.
+
+## Document format
+
+* Chinese writing guideline
+
+  Follow section 3 in https://mirrors.ustc.edu.cn/anthon/aosc-l10n/zh_CN_l10n.pdf
+
+* reStructureText syntax
+
+  Make sure `sphinx-build` run without warnings.
+
+## Credit
+
+For those improved the documnets, the Github username will be mentioned in the document.