Add CONTRIBUTING.md

Author: be-hase

CONTRIBUTING.md

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+## How to contribute to LINE Bot SDK for Python project
+
+First of all, thank you so much for taking your time to contribute! LINE Bot SDK for Python 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/line/line-bot-sdk-python/issues) to report bugs and propose new features and
+  improvements.
+- Ask a question using [the issue tracker](https://github.com/line/line-bot-sdk-python/issues).
+- Contribute your work by sending [a pull request](https://github.com/line/line-bot-sdk-python/pulls).
+
+### Contributor license agreement
+
+If you are sending a pull request and it's a non-trivial change beyond fixing typos, please make sure to sign
+[the ICLA (individual contributor license agreement)](https://feedback.line.me/enquete/public/919-h9Yqmr1u). Please
+contact us if you need the CCLA (corporate contributor license agreement).
+