Slack link no longer valid

[tripflex]

Slack link in README.md is outdated and no longer works:

This link is no longer active

To join this workspace, you’ll need to ask the person who originally invited you for a new link.

[awalker4]

Sorry about that! Another stale part of the README. We now have a shortened link that aliases to the latest slack invite:

https://short.unstructured.io/pzw05l7 (Pulled from the bottom of our homepage)