include LICENSE file in published crates

[decathorpe]

The MIT license requires that published / redistributed sources include a copy of the license text. This isn't the case for source code of the k9 crate that is published / redistributed via https://crates.io.

This PR adds a symlink in the k9 directory that points at the LICENSE file in the repository root, which makes cargo include the file when packaging / publishing the crate.

Note that if you are publishing this crate from an operating system which doesn't support symbolic links (Windows, I'm looking at you), a full copy of the file might be needed in the k9 directory, instead, to have cargo pick it up correctly.

[aaronabramov]

thank you!
(sorry i totally missed the email notification)