Add CHANGELOG.md and release version 0.1.0.

Signed-off-by: Daira Emma Hopwood <daira@jacaranda.org>

CHANGELOG.md

@@ -0,0 +1,23 @@
+# 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/1.0.0/),
+and this project adheres to Rust's notion of
+[Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [0.1.0] - 2023-03-09
+### Initial release.
+This crate contains a copy of the "pdqsort" (pattern-defeating quicksort) implementation
+from Rust 1.56.1. This is an unstable sort, i.e. the order in the output of elements that
+compare equal is not specified. In practice this order differed (for given input) between
+platforms with 32-bit `usize` and those with larger (64-bit) `usize`, and may also differ
+in future versions of Rust. If an application has mistakenly used unstable sorting with
+non-unique elements in a context where determinism was required, this crate allows the
+Rust 1.56.1 behaviour on 64-bit platforms to be replicated in other versions of Rust, and
+on 32-bit platforms.
+
+The differences from the upstream Rust standard library code are:
+* to remove use of unstable or standard-library internal Rust features that would prevent
+  compilation as an external crate.
+* to delete unused code (`partition_at_index_loop` and `partition_at_index`).
+* to add tests (based on those in the standard library) that test this crate specifically.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "halo2_legacy_pdqsort"
-version = "0.0.0"
+version = "0.1.0"
 authors = [
     "Daira Emma Hopwood <daira@jacaranda.org>",
     "Sean Bowe <sean@electriccoin.co>",