Update readme.md

Merge pull request OpenGATE#326 from dsarrut/towards-9.0

update version, log and authors

Merge branch 'develop' of https://github.com/OpenGATE/Gate into develop

add some minimal GateRTion info

admit that the 8.1 release had a version number mishap

release date is today, text cleanup

oops: version number in --version option output

Update README

GATE source code version 7.2