Add LICENSE file

[martinling]

Hi @courvoif,

We've been using your pcap-file crate in Packetry and we're very happy with it. As part of our release preparations, I've been going through the license requirements for all of our dependencies, including byteorder_slice.

Currently byteorder_slice declares an MIT license type in Cargo.toml, but there's no license file in the repository, nor in its released crates.

The terms of the MIT license require us to include both your original copyright notice, and the original license text, when we distribute our program. Because these are missing we can't technically comply with that requirement.

This PR adds the necessary LICENSE file, using the same text as the one you have for pcap-file. I have set the copyright dates based on the commits in this repository.

[vlrpl]

We are currently in the process of packaging our tool, Retis, for Fedora, which depends on pcap-file.

Since both byteorder_slice and pcap-file are not yet part of Fedora, we have submitted a review request to package them.

The MIT license requires that redistributed sources contain a copy of the license text. This also applies to crates distributed via crates.io and packages for linux distros.
For this reason, we're interested in this PR and that the license text is included in a published crate.