Add Node Slack community to support options

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Adds the Node.js Slack Community to the list of support resources. Admins from the Slack community have agreed to posting and upholding the CoC and collaborating with the Moderation Team to uphold the guidelines. cc @AlexTes @ljharb

To note: would like to try this as well with #nodejs on Freenode IRC, but it takes a bit more work. So it stays listed in unofficial until/if we're able to confirm the same and have an admin team with bandwidth to support the existing community.