doc: add releasenotes for 7.7
doc: add releasenotes for 7.7

sharang108/checkstyle,liscju/checkstyle,rnveach/checkstyle,romani/checkstyle,checkstyle/checkstyle,checkstyle/checkstyle,jochenvdv/checkstyle,ilanKeshet/checkstyle,rnveach/checkstyle,sabaka/checkstyle,jochenvdv/checkstyle,liscju/checkstyle,sabaka/checkstyle,checkstyle/checkstyle,liscju/checkstyle,ilanKeshet/checkstyle,jochenvdv/checkstyle,rnveach/checkstyle,romani/checkstyle,sharang108/checkstyle,romani/checkstyle,jonmbake/checkstyle,checkstyle/checkstyle,jonmbake/checkstyle,rnveach/checkstyle,rnveach/checkstyle,checkstyle/checkstyle,romani/checkstyle,sabaka/checkstyle,ilanKeshet/checkstyle,jonmbake/checkstyle,sharang108/checkstyle,checkstyle/checkstyle,romani/checkstyle,romani/checkstyle,rnveach/checkstyle