Update changelog (#1004)

This commit adds an unreleased section to the changelog
to track things that need to be released. It also lists the
unreleased changes since the last release to this section.

CHANGELOG.md

@@ -1,4 +1,15 @@
 # Changelog
+All notable changes to this project will be documented in this file.
+
+This project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
+
+## Unreleased
+### Fixed
+* [#1001][]: JSON: Fix complex number encoding with negative imaginary part. Thanks to @hemantjadon.
+* [#1003][]: JSON: Fix inaccurate precision when encoding float32.
+
+[#1001]: https://github.com/uber-go/zap/pull/1001
+[#1003]: https://github.com/uber-go/zap/pull/1003
 
 ## 1.19.0 (9 Aug 2021)