Add LICENSE file

[Un1q32]

Adding a license file in the root of the repo would make it easier for packagers to include the license in their package.

[xplshn]

Good, I'd go with ISC/MIT/3BSD/2BSD/0BSD :)

[Un1q32]

It's already ISC, the license is at the top of source files, I just want the standalone LICENSE file to make packaging a bit easier.