docs: add changelog

Author: gamemaker1

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/1.0.0/),
+and this project adheres to
+[Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [1.0.0](https://github.com/express-rate-limit/rate-limit-memcached/releases/tag/v1.0.0)
+
+### Breaking
+
+- Rewrite the store to implement the new `Store` interface, introduced in
+  `express-rate-limit` v6.0.0.
+- Require the `del`, `get`, `set`, `add`, `incr` and `decr` functions on any
+  `memcached` client passed to the store.
+- `MemcachedStore` is now a named export, not a default export, to make it play
+  nice with the dual cjs-esm package.
+
+### Added
+
+- Add the `locations` and `config` options.
+- Store expiry time for a client in a new key, named `${prefix}expiry:${key}`.