Add a license

[balt-dev]

Hi! I was wanting to use this as a dependency for my own project, but since there's no license, this defaults to All Rights Reserved, which means that nobody can. Would you care to add one?

[leonleon33156]

这是来自QQ邮箱的假期自动回复邮件。您好，我最近正在休假中，无法亲自回复您的邮件。我将在假期结束后，尽快给您回复。