Add CHANGELOG.md

Signed-off-by: Sandro-Alessio Gierens <sandro@gierens.de>

Author: gierens

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+## [0.12.5] - 2023-08-25
+
+This is the first version of jzon which continues the unmaintained
+json v0.12.4.
+
+### Features
+- Implement `IntoIterator` for `Object`
+* Add `as_object()` and `as_array()` methods
+
+### Bug Fixes
+* Avoid zero initialization in `Short`
+
+### Refactor
+* Update lifetimes of `read_complex_string`
+* Fix a few typos
+
+### Documentation
+- Rename to jzon
+- Add this changelog
+
+### CI
+- Add Github workflows