Community communication tool

Created by: stephanemagnenat

In the past we were using mailing lists on gna, but they shut down some months ago.

@Cor3ntin has suggested to set up a new communication tool, he mentioned riot.im or Slack. We could also set-up again mailing lists, Eigen for example uses tuxfamily.

I think that community members should express their needs and then we'll be in good position to select the right tool.