Add a README

Author: cuviper

README.md

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+# Equivalent
+
+[![crates.io](https://img.shields.io/crates/v/equivalent.svg)](https://crates.io/crates/equivalent)
+[![docs](https://docs.rs/equivalent/badge.svg)](https://docs.rs/equivalent)
+
+`Equivalent` and `Comparable` are Rust traits for key comparison in maps.
+
+These may be used in the implementation of maps where the lookup type `Q`
+may be different than the stored key type `K`.
+
+* `Q: Equivalent<K>` checks for equality, similar to the `HashMap<K, V>`
+  constraint `K: Borrow<Q>, Q: Eq`.
+* `Q: Comparable<K>` checks the ordering, similar to the `BTreeMap<K, V>`
+  constraint `K: Borrow<Q>, Q: Ord`.
+
+These traits are not used by the maps in the standard library, but they may
+add more flexibility in third-party map implementations, especially in
+situations where a strict `K: Borrow<Q>` relationship is not available.
+
+## License
+
+Equivalent is distributed under the terms of both the MIT license and the
+Apache License (Version 2.0). See [LICENSE-APACHE](LICENSE-APACHE) and
+[LICENSE-MIT](LICENSE-MIT) for details. Opening a pull request is
+assumed to signal agreement with these licensing terms.