Add LICENSE for the project

[chenrui333]

👋 It looks like right now there is no license for the project, please consider adding one. :)

[oscartbeaumont]

It would be great if the licence was properly added into a LICENCE file in the root of the repository but it has been declared as WTFPL here.

[NukaCody]

As a heads up, the project license is different than the actual ffmpeg license which is LGPL and GPL. Creating bindings to a library still means your subject to the underlying library license. You choose which license via a feature flag https://github.com/zmwangx/rust-ffmpeg-sys/blob/master/build.rs#L278