Updated README for v4.0, styled Slack Login Page

Co-authored-by: Sonny Nguyen <sonnynguyen163@gmail.com>
Co-authored-by: Jeffrey Sul <jeffreydsul@gmail.com>
Co-authored-by: Kenneth Lee <kennyea87@gmail.com>
Co-authored-by: Ryan Bender <rdbender@me.com>

Author: 5 people