Add license files

Proposal

As written in the documentation and manifests, arti is licensed under MIT or Apache-2.0. However, as far as I can see, corresponding license files are not present in the repository. So what I'm proposing is: it would be good to have e.g. LICENSE-MIT and LICENSE-APACHE files at the root of the repository, very much like the Rust project itself has.

Additional Details

I'm an Arch Linux maintainer and I recently created a package in the AUR: https://aur.archlinux.org/packages/arti/

The problem is, the Apache license is installed on the system via licenses package as default. However, for the MIT license, packagers should distribute the license along with the package. (as mentioned in the ArchWiki) Because of the missing license files, I cannot include them in the package properly thus I have to use a dirty hack.