Update Slack Shield

[MooooCat]

Update Slack Shield to Readme.md

Description

Readers can now join the SDG community through Slack by clicking the Slack Join button on the homepage.

Motivation and Context

Readers can now join the SDG community via Slack

How has this been tested?

Not Applicable

Types of changes

• Maintenance (no change in code, maintain the project's CI, docs, etc.)
• 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 not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.