Add missing Unicode license text

[michel-slm]

(taken from https://www.unicode.org/license.txt)

[decathorpe]

It could also be argued that the license in crate metadata should be (MIT OR Apache-2.0) AND Unicode-DFS-2016, similar to what some other crates are doing (like https://crates.io/crates/unicode-ident), though there is some disagreement about the exact semantics of the package.license field in Cargo.toml (see rust-lang/regex#933, among others).