Prepare release 3.3.1

Prepare release 3.3.0

README.md: add recommendation to use subscriber index

3.1.2-RC: minor version updates, etc.

README.md: set gradle dependency to 3.1.1

3.1.1 / 3.1.0

updated README.md for 3.1.0-RC

Merge branch 'annotations'

V2.4.0, updated docs