deps: describe the crate and dual-license it MIT or GPL-2.0-only

Publishing to crates.io requires a description and a license; the
repository and readme give the crates.io page the right links. The
license texts ship as LICENSE-MIT and LICENSE-GPL.

Dual licensing is valid while no lintian-derived code is in tree:
shelling out to lintian is mere aggregation. When derived lint
collections are implemented they must land in a separate GPL-2-only
crate, since their copyright belongs to lintian's authors and
cannot carry an MIT grant; the core crate stays dual.

Cargo.toml

@@ -3,6 +3,10 @@ name = "pkh"
 version = "0.1.0"
 edition = "2024"
 authors = ["vhaudiquet"]
+description = "pkh is a packaging helper for Debian/Ubuntu packages"
+license = "MIT OR GPL-2.0-only"
+repository = "https://git.vhaudiquet.fr/vhaudiquet/pkh"
+readme = "README.md"
 
 [dependencies]
 clap = { version = "4.5.51", features = ["cargo"] }