(doc) Fixing link to Community Slack

[jst-cyr]

Summary

README updated. The URL for the community Slack has been updated to correctly point to https://puppetcommunity.slack.com/ instead of https://slack.puppet.com

[bastelfreak]

@jst-cyr @pmcmaw I don't think this change is correct. puppetcommunity.slack.com always was the login page. You cannot register there. slack.puppet.com redirected to a register page.

[jst-cyr]

@bastelfreak : I agree, it's not an equivalent fix. The slack.puppet.com URL doesn't work right now, though, so I was dropping in a quick fix to get a working URL again.

On the puppet.com website, we use this: https://join.slack.com/t/puppetcommunity/shared_invite/zt-30zbgx1ti-wRR9Et_K~Nt~D62jLixWcw

That takes to a registration page and may be a better destination, but it's probably better to have that centrally managed behind a redirect instead of in every README.

[jst-cyr]

@pmcmaw and @bastelfreak : I've just been notified by IT that my request to get slack.puppet.com working again as a join link has been completed. I'll submit a new PR to get us reverted back to the old URL now that it works!