Enhance README with marketplace link and improved formatting

[Copilot]

This PR enhances the repository's README.md file to provide a more professional and informative introduction to the CodeRabbit VSCode extension public repository.

Changes Made

• Improved title: Changed from generic "vscode-extension" to descriptive "CodeRabbit VSCode Extension"
• Enhanced formatting: Fixed missing space and improved overall readability
• Added marketplace link: Included the Visual Studio Code Marketplace installation link as requested
• Better structure: Organized content into Installation and Support sections
• Clearer messaging: Maintained the core purpose (issues and discussions) while making it more user-friendly

Before

# vscode-extension
Public repo for the CodeRabbit VSCode extension(Issues and Discussions)

After

The README now includes:

• Clear extension name and purpose
• Direct marketplace installation link: https://marketplace.visualstudio.com/items?itemName=CodeRabbit.coderabbit-vscode
• Organized sections for Installation and Support
• Links to Issues section for bug reports and feature requests

This provides users with immediate access to install the extension while clearly communicating that this repository is the hub for community feedback and discussions.

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