Add slack to support channels

[MPParsley]

There's now an #og channel on drupal.slack.com
See slack://channel?team=T06GX3JTS&id=CELR48EA0

[pfrenssen]

Why not use the HTTP link to the chat page? https://drupal.slack.com/messages/CELR48EA0/

This way it is accessible also by people who choose not to install the Slack application (which is not open source).

[MPParsley]

Why not use the HTTP link to the chat page? https://drupal.slack.com/messages/CELR48EA0/

Good point. The reason I used the slack protocol was:

• because we can (not so much though :-) )
• seems like we do the same for irc (where we could also add a link to freenode instead)

edit: you're right it's not open source, I added the http link.