Move from Gitter to CNCF Slack

[ocelotl]

The community seems to be moving away from Gitter and into Slack for DMs and discussions. We may want to follow the same path.

There are several opentelemetry-related channels already at Slack. We could open # otel-python too.

Now, in my personal opinion, I would prefer to move to Slack. I find Gitter less feature-rich and a bit buggy. I also use Slack already for work so, I would prefer to have only one tool for messaging.

We can discuss this in the next SIG, in the meantime please leave comments below with your thoughts.

[lzchen]

Work items:

Remove "Gitter" from our docs.
Update CONTRIBUTING.md, and update with process outlined here

[codeboten]

The channel exists here: https://cloud-native.slack.com/archives/C01PD4HUVBL