Add change log, resolves #90

Author: vearutop

CHANGELOG.md

@@ -0,0 +1,18 @@
+# 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).
+
+## [0.12.21] - 2019-10-01
+
+### Fixed
+- Treating unresolvable schema as a prohibitive `false` schema instead of failing with exception.
+
+## [0.12.20] - 2019-09-22
+
+### Changed
+- Export `null` value instead of skipping it for properties having `null` type.
+
+[0.12.21]: https://github.com/swaggest/php-code-builder/compare/v0.12.20...v0.12.21
+[0.12.20]: https://github.com/swaggest/php-code-builder/compare/v0.12.19...v0.12.20