See https://doc.rust-lang.org/cargo/reference/manifest.html#the-license-and-license-file-fields :
> If a package is using a nonstandard license, then the license-file field may be specified in lieu of the license
> field.
`simple_asn1` however uses a standard license (ISC, as per LICENSE).
Note that this is also a somewhat commonly used way by many projects
(see for example Cargo or serde).
Using the machine-readable SPX specifier instead of a text file helps
automatic tooling like [cargo-lichking](https://github.com/Nemo157/cargo-lichking) to check licenses of projects and
dependencies.
