Introduce a changelog

Author: shepmaster

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [2.0.0] - Unreleased
+
+[2.0.0]: https://github.com/shepmaster/twox-hash/tree/v2.0.0
+
+This release is a complete rewrite of the crate, including
+reorganization of the code. The XXH3 algorithm now matches the 0.8
+release of the reference C xxHash implementation.
+
+### Added
+
+- `XxHash32::oneshot` and `XxHash64::oneshot` can perform hashing with
+  zero allocation and generally improved performance. If you have code
+  that creates a hasher and hashes a slice of bytes exactly once, you
+  are strongly encouraged to use the new functions. This might look
+  like:
+
+  ```rust
+  // Before
+  let mut hasher = XxHash64::new(); // or XxHash32, or with seeds
+  some_bytes.hash(&mut hasher);
+  let hash = hasher.finish();
+
+  // After
+  let hash = XxHash64::oneshot(some_bytes);
+  ```
+
+- There is a feature flag for each hashing implementation. It is
+  recommended that you opt-out of the crate's default features and
+  only select the implementations you need to improve compile speed.
+
+### Changed
+
+- The crates minimum supported Rust version (MSRV) is now 1.81.
+
+- Functional and performance comparisons are made against the
+  reference C xxHash library version 0.8.2, which includes a stable
+  XXH3 algorithm.
+
+- Support for randomly-generated hasher instances is now behind the
+  `random` feature flag. It was previously combined with the `std`
+  feature flag.
+
+### Removed
+
+- The deprecated type aliases `XxHash` and `RandomXxHashBuilder` have
+  been removed. Replace them with `XxHash64` and
+  `xxhash64::RandomState` respectively.
+
+- `RandomXxHashBuilder32` and `RandomXxHashBuilder64` are no longer
+  available at the top-level of the crate. Replace them with
+  `xxhash32::RandomState` and ``xxhash64::RandomState` respectively.
+
+- `Xxh3Hash64` and `xx3::Hash64` have been renamed to `XxHash3_64` and
+  `xxhash3_64::Hasher` respectively.
+
+- The free functions `xxh3::hash64`, `xxh3::hash64_with_seed`, and
+  `xxh3::hash64_with_secret` are now associated functions of
+  `xxhash3_64::Hasher`: `oneshot`, `oneshot_with_seed` and
+  `oneshot_with_secret`. Note that the argument order has changed.
+
+- Support for the [digest][] crate has been removed. The digest crate
+  is for **cryptographic** hash functions and xxHash is
+  **non-cryptographic**.
+
+- `XxHash32` and `XxHash64` no longer implement `Copy`. This prevents
+  accidentally mutating a duplicate instance of the state instead of
+  the original state. `Clone` is still implemented so you can make
+  deliberate duplicates.
+
+- The XXH3 128-bit variant is not yet re-written. Work is in progress
+  for this.
+
+- We no longer provide support for randomly-generated instances of the
+  XXH3 64-bit variant. The XXH3 algorithm takes both a seed and a
+  secret as input and deciding what to randomize is non-trivial and
+  can have negative impacts on performance.
+
+[digest]: https://docs.rs/digest/latest/digest/