Add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+## Pull Request Process
+
+When you want to contribute to this project,
+please first discuss the change you wish to make via GitHub Issue.
+
+1. Check that your patch is legal under the [LICENCE](https://github.com/qulacs/qulacs/blob/master/LICENSE).
+2. Test and format in your computer.
+    - If you want to know how to do it please see `How to format and test` section below.
+3. PR to the `dev` instead of `master` branch.
+    - Because we use `master` branch as the release branch.
+
+## How to format and test
+
+You should format and test in your computer before you submit a PR.
+First you have to install [dependecies](https://github.com/qulacs/qulacs#requirement) and [clang-format](https://clang.llvm.org/docs/ClangFormat.html) (version 9) to your computer
+before you build and test your patch.
+And you execute these commands at the root directory of this project:
+
+### For macOS and Linux
+
+```console
+$ git ls-files "*.cpp" | xargs clang-format -style=file -i
+$ ./script/build_gcc.sh
+$ python setup.py install
+$ cd build
+$ make test
+$ make pythontest
+```
+
+### For Windows
+
+```console
+$ git ls-files "*.cpp" | xargs clang-format -style=file -i
+$ ./script/build_msvc_2015.bat
+$ python setup.py install
+$ cmake --build ./visualstudio --target test --config Release
+$ cmake --build ./visualstudio --target pythontest --config Release
+```
+
+### Tips
+
+- We only check the format in Linux. Travis CI could point out format errors even if there is no error in your Windows or macOS.
+- CI requires `clang-format` version 9 so you should use the same version of `clang-format` to avoid some trouble.
+- We recommend use Python from [pyenv](https://github.com/pyenv/pyenv) and [vertualenv](https://pypi.org/project/virtualenv/) for the local test.
+    - Since we run `python setup.py install` at global Python unstable qulacs would be installed unintentionally.
+    - And it would be difficult to show dependencies version when we need for debug