chore: add CHANGELOG entry for 2.0.0-alpha.0 (#12013)

Keep the text simple. Calling out the list of changes is not important
for the first release. Subsequent releases will carry the standard
format of features and bug fixes.


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CHANGELOG.v2.md

@@ -1,3 +1,7 @@
 # Changelog
 
 All notable changes to this project will be documented in this file. See [standard-version](https://github.com/conventional-changelog/standard-version) for commit guidelines.
+
+## 2.0.0-alpha.0 (2020-12-11)
+
+This is the first alpha release of CDK 2.0. 🎉