update changelog - mention AndroidX please

[VividWombat]

for those of us supporting pre-AndroidX projects, its important to know that 2.0.1 has moved to using AndroidX. Android Studio suggests upgrading from 2.0.0 to 2.0.1, which will break the project.

suggest making a note in the changelog. BREAKING CHANGE - migrate to AndroidX.

might have been better to push the version to 3.0.0. maybe something for next version.