Fix missing LICENSE files in published crates

[musicinmybrain]

The chosen Apache-2.0 license requires copies of the license text to be distributed with the software, but the published crates do not include it. This PR fixes that by adding symbolic links to the top-level LICENSE file, which are resolved during crate publication.

Before this PR:

$ git clone https://github.com/CycloneDX/cyclonedx-rust-cargo.git
$ cd cyclonedx-rust-cargo
$ cd cyclonedx-bom
$ cargo publish --dry-run
$ tar -tzvf ../target/package/cyclonedx-bom-0.8.0.crate | grep LICENSE
[no output]

After this PR:

$ cargo publish --dry-run
$ tar -tzvf ../target/package/cyclonedx-bom-0.8.0.crate | grep LICENSE
-rw-r--r-- 0/0           11341 2006-07-24 02:21 cyclonedx-bom-0.8.0/LICENSE

[lfrancke]

Thank you for the contribution!