Add Apache License

[ryouta-hiyoshi]

I would like to use this library but I noticed there is no license display.
Could you add any license to this repository to clarify your copyrights?
I looked at your other repositories and noticed that some of them are Apache licensed. It would be a great help to me if you could provide this library under that.

[webbrain-one]

Hi! Thanks for the suggestion. I've just opened a pull request to address this: #3. It adds the Apache 2.0 license file to the repository, which should clarify the copyright terms and match your other projects. Please take a look when you get a chance! 😊