Add CSV header requirements and examples to README

[Copilot]

This PR adds comprehensive documentation for the CSV format requirements that users need to follow when uploading data to the GitHub Stats Visualizer.

Changes Made

Added a new "CSV Format Requirements" section to the README that includes:

1. Complete list of required headers - All 27 required CSV columns with exact naming:

   Org_Name,Repo_Name,Is_Empty,Last_Push,Last_Update,Is_Fork,Is_Archived,Repo_Size_MB,Collaborator_Count,Protected_Branch_Count,Milestone_Count,Issue_Count,Pull_Request_Count,PR_Review_Count,PR_Review_Comment_Count,Commit_Comment_Count,Issue_Comment_Count,Issue_Event_Count,Release_Count,Project_Count,Branch_Count,Tag_Count,Discussion_Count,Has_Wiki,Repo_URL,Migration_Issue,Created
   

2. Detailed header descriptions table - Explaining the data type and purpose of each field (string, number, boolean, ISO 8601 timestamps, etc.)

3. Sample CSV row - A realistic example showing proper data format with actual values

4. Important notes - Clarifying that all headers must be present for successful parsing

Validation

• ✅ Generated sample CSV using the existing generateTestData utility
• ✅ Successfully uploaded and analyzed sample data in the application
• ✅ Confirmed all charts and statistics display correctly
• ✅ Verified the application builds without errors

Screenshot

The updated documentation now provides users with everything they need to prepare their GitHub repository data for upload and analysis:

This resolves the issue where users didn't know what CSV headers were expected by the application, eliminating parsing errors and improving the user experience.

Fixes #1.

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[amenocal]

LGTM