Update Slack instructions on the README

[wk8]

Describe the bug
The README says that anyone can join your Slack channel (https://github.com/uber/kraken#contact). However, it seems that your Slack channel is not configured to accept sign-ups from non-uber.com email addresses, making it impossible for someone outside of your organization to join.

To Reproduce
Steps to reproduce the behavior: follow the link from https://github.com/uber/kraken#contact, then try to sign up with a generic email address.

Expected behavior
I'd like to be able to reach out to maintainers. :)

[tommyalatalo]

+1 on this, the documentation is lacking for the project and there is no way to get a hold of anyone to ask

[ish-xyz]

+1 is this project dead? There's a poor documentation and I can't login into your slack.

[ish-xyz]

@codygibb Maybe you can help with this?

[dweekly]

The Slack channel link in the README at https://uber-container-tools.slack.com/join/shared_invite/enQtNTIxODAwMDEzNjM1LWIwYzIxNmUwOGY3MmVmM2MxYTczOTQ4ZDU0YjAxMTA0NDgyNzdlZTA4ZWVkZGNlMDUzZDA1ZTJiZTQ4ZDY0YTM#/shared-invite/error now appears fully broken, not just restricted to Uber employees.