Add 3.0 and 2.3 changes to CHANGELOG.md

We switched over to using the GitHub release notes on version 2.3 and later.

This adds references to the GitHub release notes in the CHANGELOG.md

Author: goneall

CHANGELOG.md

@@ -1,9 +1,13 @@
 # Change Log
 All notable changes to this project will be documented in this file.
 
-## 3.0 (TBD)
+## 3.0
 
-* Refactored the build process and mkdocs process.
+See the [v3.0 GithHub release notes](https://github.com/spdx/spdx-spec/releases/tag/v3.0) for changes.
+
+## 2.3
+
+See the [v2.3 GithHub release notes](https://github.com/spdx/spdx-spec/releases/tag/v2.3) for changes.
 
 ## 2.2 (2020-05-02)