Initial CHANGELOG.md (#190)

* Create CHANGELOG.md

WIP - create initial changelog based on historical release notes (https://firebase.google.com/support/release-notes/admin/node#5.8.0)

* Update CHANGELOG.md

Fixed remaining formatting

* Update CHANGELOG.md

Added link to public release notes

* Update CHANGELOG.md

Actually link to reference docs

* Update CHANGELOG.md

Addressed feedback from Hiranya

Author: teskey