docs: contribution guidelines

CONTRIBUTING.md

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+# Contribution guidelines
+
+First of all, thanks for taking the time to contribute!
+
+Please refer to the following guidelines to contribute new functionality or bug fixes:
+
+1. Use [autopep8](https://github.com/hhatto/autopep8) to format the Python code.
+2. Use [clang-format](https://clang.llvm.org/docs/ClangFormat.html) to format C++ code. Changes to BytePS C++ code should conform to [Google C++ Style Guide](https://google.github.io/styleguide/cppguide.html).
+3. Add unit tests for any new code you write.
+4. Run unit tests in both CI and GPU environments.