Later versions of Cinemachine (2.4+) have release notes installed with the package. Display the Cinemachine About panel (navigate to Cinemachine | About), then view the release notes:
Figure 11.22 – The built-in About page of the Cinemachine suite of tools
You can also find the documentation online at https://docs.unity3d.com/Packages/com.unity.cinemachine@2.1/manual/index.html.