Add changelog for 3.0.0 (#8)

Co-authored-by: Michael Weigelt <michael.weigelt@dfinity.com>

CHANGELOG.md

@@ -6,6 +6,20 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 
+## 3.0.0 - 2024-02-06
+
+### Added
+- New endpoint `/api.json` that serves an OpenAPI documentation of the PocketIC server.
+- Instances can be created from existing NNS state.
+
+### Changed
+- Breaking: The create_instance endpoint accepts an ExtendedSubnetConfigSet, which allows more options. 
+
+### Fixed
+- Canister inspect message errors when executing ingress messages are returned as canister execution results rather than request errors.
+- Subnets agree on which subnet id is the NNS subnet id. Fixes the problem where a canister installation via CMC directly would fail. 
+
+
 ## 2.0.1 - 2023-11-23
 
 ### Fixed