include LICENSE.txt in all published crates

While a LICENSE.txt is present in the toplevel directory, running "cargo publish" in workspace members will not see it, and it will not be included in published crates (e.g. on crates.io).

Assuming you're running cargo publish on a UNIX-like platform, adding symlinks from all crate directories in the workspace to the LICENSE.txt in the repository root is enough to have cargo publish pick it up.