Add open source license

[kamui-fin]

It seems like the project doesn't contain a license, which has some significant consequences on how the project can be used:

However, without a license, the default copyright laws apply, meaning that you retain all rights to your source code and no one may reproduce, distribute, or create derivative works from your work. If you're creating an open source project, we strongly encourage you to include an open source license

Please consider adding an MIT license or similar. Thank you!