Update of website text for getting slack invitations

[Sebastian]

Hi,

Currently, the documentation at 
http://mxnet.io/community/mxnet_channels.html states that people need to 
send a mail to d...@mxnet.apache.org to get an invite for slack.

It would be great if someone could update the text and ask people to 
subscribe to the dev mailing first, before sending mails, as we have to 
manually moderate the mails if people did not subscribe.

It would also be good for the project if everyone who wants to join the 
conversation on slack also subscribes to dev@ as the mailinglists play 
an important role in Apache projects (as many official decisions get 
made there).

Best,
Sebastian