feat(services): Remove Slack

Our Slack documentation has been moved to Google Docs and will be added to the onboarding Trello.
This change encourages inclusiveness, as not every member of our team is familiar with GitHub.

Author: catheraaine

services/README.md

@@ -11,6 +11,5 @@ The services we use. Some we pay for, others we mooch off. We love them all.
 * **[Harvest](harvest)**
 * **[InVision](invision)**
 * **[SendGrid](sendgrid)**
-* **[Slack](slack)**
 * **[Jira](jira)**
 * **[Mailchimp](mailchimp)**