Update readme with up to date release planning
Update readme with up to date release planning

Also mentions compatibilities I want to see maintained.
These might still change though, depending on feedback from
the community.

ryanfox1985/acts-as-taggable-on,danielma/acts-as-taggable-on,EndingChaung/acts-as-taggable-on,onursarikaya/acts-as-taggable-on,carpeliam/acts-as-taggable-on,ches/acts-as-taggable-on,Antiarchitect/acts-as-taggable-on,Vorob-Astronaut/acts-as-taggable-on,JakeTheSnake3p0/acts-as-taggable-on,AlexVPopov/acts-as-taggable-on,FlowerWrong/acts-as-taggable-on,sideci-sample/sideci-sample-acts-as-taggable-on