Add details & workflow steps to Quickstart guide

[chriswahl]

Description

This PR adds some more details to the Quickstart guide to address the username change in Slack and also provide info that you need to use @username when setting up the bot admins config item.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[devblackops]

Thanks @chriswahl. This is much more clear!