doc: update README for v3.0 (#48)
doc: update README for v3.0 (#48)

* add `release/v2.x` to branch table
* doc: adjust README to single main line 'master' branch strategy

Co-authored-by: Tobias Nett <cde1be4ebe2bbdfc1f1c71fd04cc97278e8cdd13@googlemail.com>
MovingBlocks/TeraNUI