Introduce CLA(Contributor License Agreements) for external contributors.

[romandev]

We might need CLA for contributors to ensure that once someone has provided a contribution, you cannot try to withdraw permission for its use at a later date.

• https://cla.github.com/
• https://cla.developers.google.com/

[yjaeseok]

I will try this issue

[yjaeseok]

@romandev Please add CLA file in your gist. Because CLA file must be added in personal account gist, I think your gist (owners gist) is better than mine.

[yjaeseok]

After then I will add CLA badge in README.md file 👍