Prepare clippy_utils README and Cargo.toml for publishing (#13693)

Follow up to #13691

Adds metadata to the `clippy_utils/Cargo.toml`, which is mostly copied
from the root `Cargo.toml`.
Adds a `README.md` file listing the nightly version `clippy_utils` can
be used with, mentions that there are no stability guarantees and the
license.

The next PR will add automation to update the nightly toolchains in
those files and the versions in the `Cargo.toml`s.

cc #13556

changelog: none

Author: blyxyas

clippy_utils/Cargo.toml

@@ -2,6 +2,12 @@
 name = "clippy_utils"
 version = "0.1.84"
 edition = "2021"
+description = "Helpful tools for writing lints, provided as they are used in Clippy"
+repository = "https://github.com/rust-lang/rust-clippy"
+readme = "README.md"
+license = "MIT OR Apache-2.0"
+keywords = ["clippy", "lint", "utils"]
+categories = ["development-tools"]
 
 [dependencies]
 arrayvec = { version = "0.7", default-features = false }

clippy_utils/README.md

@@ -0,0 +1,40 @@
+# `clippy-utils`
+
+Helpful tools for writing lints, provided as they are used in Clippy.
+
+## Usage
+
+This crate is only guaranteed to build with this `nightly` toolchain:
+
+<!-- begin autogenerated nightly -->
+```
+nightly-2024-11-14
+```
+<!-- end autogenerated nightly -->
+
+To use `clippy-utils` in your lint, add the following to your `Cargo.toml`:
+
+```
+clippy_utils = "0.1.XY"
+```
+
+`XY` is the version of the nightly toolchain above and can be determined with `rustc +nightly-YYYY-MM-DD -V`.
+
+## :warning: Stability :warning:
+
+No stability guarantees are made for this crate! Use at your own risk.
+
+Function signatures can change or be removed without replacement without any prior notice.
+
+## LICENSE
+
+<!-- REUSE-IgnoreStart -->
+
+Copyright 2014-2024 The Rust Project Developers
+
+Licensed under the Apache License, Version 2.0
+<[https://www.apache.org/licenses/LICENSE-2.0](https://www.apache.org/licenses/LICENSE-2.0)> or the MIT license
+<[https://opensource.org/licenses/MIT](https://opensource.org/licenses/MIT)>, at your option. Files in the project may
+not be copied, modified, or distributed except according to those terms.
+
+<!-- REUSE-IgnoreEnd -->