Add Slack Channel for k8satl meetup

[phenixblue]

This PR is to add a dedicated Slack Channel (meetup-atlanta) for the Kubernetes Atlanta Meetup group. This will help facilitate general communication with Meetup members, speakers, etc. and foster a more robust community. This also adds a Slack Usergroup (k8satl-hosts) to make it easy for users to address the Meetup hosts to recommend speakers, topics, and other related items.

Relevant Links

• Meetup Group: https://www.meetup.com/Kubernetes-Atlanta-Meetup/
• Meetup Youtube Channel: https://meetups.k8satl.io/videos/
• Meetup Notes: http://github.com/k8satl/meetups/

[phenixblue]

/assign @alejandrox1

[moshevayner]

/lgtm

[mrbobbytables]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mrbobbytables, phenixblue

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