Update README.md to add the Drupal project.

[sillygwailo]

Add a Slack Group

• [x] Suggestion is not a duplicate
• [x] Suggestion is placed in the proper country according the slack channel’s demographic or if it’s a general channel, it’s placed in the appropriate category
• [x] Name and link are accurate
• [x] Brief description of the channel is accurate and properly typed

Remove a Slack Group

• [ ] Name of Slack group is present
• [ ] Reason for requesting removal is present