exp: Add a changelog (#1353)

Backfills a changelog for the releases of exp so far.

exp/CHANGELOG.md

@@ -0,0 +1,18 @@
+# Changelog
+All notable changes to this project will be documented in this file.
+
+This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## 0.2.0 - 9 Sep 2023
+
+Breaking changes:
+* [#1315](https://github.com/uber-go/zap/pull/1315): zapslog: Drop HandlerOptions.New in favor of just the NewHandler constructor.
+* [#1320](https://github.com/uber-go/zap/pull/1320), [#1338](https://github.com/uber-go/zap/pull/1338):
+  zapslog: Drop support for golang.org/x/exp/slog in favor of log/slog released in Go 1.21.
+
+## 0.1.0 - 1 Aug 2023
+
+Initial release of go.uber.org/zap/exp.
+This submodule contains experimental features for Zap.
+Features incubated here may be promoted to the root Zap module,
+but it's not guaranteed.