docs: add monorepo structure description to README

[devin-ai-integration]

Add monorepo structure description to README

This PR adds a simple description of the repository's monorepo structure to the README.md file to help new contributors understand the project organization better.

Changes

• Added a note about the monorepo structure and pnpm workspaces management
• Placed the description at the start of the "In this repo..." section for visibility

Testing

• Verified README.md renders correctly with the new changes
• No functional changes, documentation only

Link to Devin run: https://app.devin.ai/sessions/ef4c20f48cc945c79648f198f456ec9d

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Name	Status	Preview	Comments	Updated (UTC)
docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Dec 16, 2024 5:53am
upstreet.ai	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Dec 16, 2024 5:53am

[devin-ai-integration]

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