Add reddit-mods Slack channel

The r/kubernetes subreddit has grown to over 45k subscribers and is
loosely maintained by a small set of moderators that largely overlap
with the Kubernetes Slack org.

This channel is being created to get our mod team organized about
policies and enforcement, and to work towards sufficient quality
and Code of Conduct enforcement to eventually become an "official"
subreddit (should the community will it).

Signed-off-by: Greg Taylor <greg@gctaylor.com>

communication/slack-config/channels.yaml

@@ -277,6 +277,7 @@ channels:
   - name: rancher
   - name: random
     archived: true
+  - name: reddit-mods
   - name: rktnetes
   - name: ro-users
   - name: ru-users