Consolidate the README and split release notes

Cargo.toml

@@ -7,7 +7,6 @@ rust-version = "1.63.0"
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/rayon-rs/either"
 documentation = "https://docs.rs/either/1/"
-readme = "README-crates.io.md"
 
 description = """
 The enum `Either` with variants `Left` and `Right` is a general purpose sum type with two cases.

README.md

@@ -0,0 +1,36 @@
+# Either
+
+[![Either crate](https://img.shields.io/crates/v/either.svg)](https://crates.io/crates/either)
+[![Either documentation](https://docs.rs/either/badge.svg)](https://docs.rs/either)
+![minimum rustc 1.63](https://img.shields.io/badge/rustc-1.63+-red.svg)
+[![build status](https://github.com/rayon-rs/either/actions/workflows/ci.yml/badge.svg)](https://github.com/rayon-rs/either/actions)
+
+The enum `Either` with variants `Left` and `Right` is a general purpose sum
+type with two cases.
+
+`Either` has methods that are similar to `Option` and `Result`, and it also
+implements traits like `Iterator`, `Read`, and `Write`.
+
+The crate includes macros `try_left!()` and `try_right!()` to use for
+short-circuiting logic, similar to how the `?` operator is used with `Result`.
+Note that `Either` is general purpose and unopinionated about the meaning of
+`Left` and `Right`. For describing success or error, use the regular `Result`.
+
+Please read the [API documentation here](https://docs.rs/either/), and see
+[RELEASES.md][RELEASES.md] for the version history.
+
+How to use with cargo:
+
+```toml
+[dependencies]
+either = "1"
+```
+
+## License
+
+Dual-licensed to be compatible with the Rust project.
+
+Licensed under the Apache License, Version 2.0
+<https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+<https://opensource.org/licenses/MIT>, at your option. This file may not
+be copied, modified, or distributed except according to those terms.