Create CONTRIBUTING.md

CONTRIBUTING.md

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+## How to contribute to Drucker project
+
+First of all, thank you so much for taking your time to contribute! Drucker is not very different from any other open
+source projects you are aware of. It will be amazing if you could help us by doing any of the following:
+
+- File an issue in [the issue tracker](https://github.com/drucker/drucker-client/issues) to report bugs and propose new features and
+  improvements.
+- Ask a question by creating a new issue in [the issue tracker](https://github.com/drucker/drucker-client/issues).
+  - Browse [the list of previously asked questions](https://github.com/drucker/drucker-client/issues?q=label%3Aquestion).
+- Contribute your work by sending [a pull request](https://github.com/drucker/drucker-client/pulls).
+
+### Contributor license agreement
+
+When you are sending a pull request and it's a non-trivial change beyond fixing typos, please sign 
+[the ICLA (individual contributor license agreement)](https://cla-assistant.io/drucker/drucker). Please
+[contact us](dl_oss_dev@linecorp.com) if you need the CCLA (corporate contributor license agreement).
+
+### Code of conduct
+
+We expect contributors to follow [our code of conduct](https://github.com/drucker/drucker-client/blob/master/CODE_OF_CONDUCT.md).