Auto merge of #7347 - matthiaskrgr:manifest_readme, r=alexcrichton

add readme key to cargos manifest.

This should make the readme display on cargos crates.io page

Author: bors

Cargo.toml

@@ -9,6 +9,7 @@ license = "MIT OR Apache-2.0"
 homepage = "https://crates.io"
 repository = "https://github.com/rust-lang/cargo"
 documentation = "https://docs.rs/cargo"
+readme = "README.md"
 description = """
 Cargo, a package manager for Rust.
 """