Include LICENSE file in the published package

[Tamschi]

Currently, cargo-about is unable to automatically include Stretch in the generated third party licenses file as it is missing both the license field in Cargo.toml and the LICENSE file in the package published to crates.io.

Including the license file in the next published version should fix this issue.

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(I'll add the Stretch license manually to my distribution package for the time being so there's no hurry. It would still be nice to get this fixed, as Stretch is the only one of 51 crates with this problem.)