Ensure license file is published with each crate

[a-wai]

As the license file is present at the root of the repository, it isn't part of any crate on crates.io, as those lie in subfolders. Ensure this file is published by telling cargo where to find it.

[10ne1]

Gentle ping?

[musicinmybrain]

This does appear to work (tested with cargo publish --dry-run), but it does produce a warning from cargo:

warning: only one of `license` or `license-file` is necessary
`license` should be used if the package license can be expressed with a standard SPDX expression.
`license-file` should be used if the package uses a non-standard license.
See https://doc.rust-lang.org/cargo/reference/manifest.html#the-license-and-license-file-fields for more information.

A widely-used alternative approach would be to use symbolic links to the top-level license file, as in #113.

[a-wai]

#113 looks better indeed, closing this one. Thanks!