add LICENSE file

[hoijui]

hey! :-)
could you maybe do this?

[shymega]

Hey.

My guile-rs is licensed under the LGPLv3, and as the two projects - that is, this one, and mine - are going to be merged, I propose licensing under the LGPLv3. I'd add a license now, but I want to allow the other authors to have their thoughts accounted for.

cc: @Javyre @cherryman