Documentation points to "closed" gitter channel

[aucampia]

from https://includeos.readthedocs.io/en/latest/Contributing-to-IncludeOS.html#gitter-chat

We are usually present in our public gitter channel for any kinds of questions.

When you click this and click join room it brings up popup reading:

IncludeOS has migrated to slack. Please join us over at: https://goo.gl/NXBVsc

That link resolves to: https://includeos.slack.com/join/shared_invite/enQtNDY4NDgxNDgzNjU1LWUzNjc5ODBjMWQ0MWUyNmJmNGVkYWIyYWEwYzU1ZmExYmEwNWU5MTM3MWZkOTQzNWJkOTRmODVkZWI4Yjg2Nzc

Which is no longer valid:

This invite link is no longer active.
If you have an @includeos.org email address, you can still create an account.
If not, please check with the person who shared the invite link with you to ask for an invitation.

It seems there is no public IM anymore - so maybe it should be removed from documentation completely?