update readme

Moved to Beta4 version

Move to Beta3 release version

Release 5.21.0

Move to version 5.20.0

Releasing 5.19.0.2

Move to 6.0.0.Beta2 release

Move to version 5.19.0.1

Merge pull request Activiti#721 from dbmalkovsky/docChanges

Minor spelling changes to User Guide

Release notes + migration docs changed for Activiti 6 Beta 1 release