README: add the Gitter and the mailing list links in the Community section
README: add the Gitter and the mailing list links in the Community section

bpollack/factor,tgunr/factor,AlexIljin/factor,bpollack/factor,mrjbq7/factor,bpollack/factor,tgunr/factor,factor/factor,AlexIljin/factor,bpollack/factor,bpollack/factor,AlexIljin/factor,mrjbq7/factor,AlexIljin/factor,mrjbq7/factor,tgunr/factor,tgunr/factor,mrjbq7/factor,factor/factor,bpollack/factor,AlexIljin/factor,AlexIljin/factor,bpollack/factor,tgunr/factor,factor/factor,AlexIljin/factor,mrjbq7/factor,factor/factor,factor/factor,mrjbq7/factor,factor/factor,tgunr/factor