docs: add comprehensive README.md for GitQuest

[Copilot]

The project had a single-line # GitQuest placeholder as its README — no setup instructions, no feature overview, nothing to orient new contributors or users.

Changes

• Replaced the stub README.md with a full project README covering:
  • Tagline, attention-hook intro, and shields.io badge row (stars, license, issues, PRs welcome, Next.js, .NET)
  • Placeholder demo banner + 3-column preview table (Discover / Leaderboard / Profile)
  • Feature list with emojis, tech stack table (Next.js 16 + ASP.NET Core 8 + SQL Server + GitHub OAuth)
  • Quick Start — single copy-paste block: clone → dotnet run → npm run dev
  • Installation — backend (appsettings.Development.json template with all required keys) + frontend commands
  • Usage — step-by-step flow with concrete API snippets (GET /api/issues/discover?language=typescript, POST /api/issues/{issueId}/claim)
  • "Why GitQuest?" motivation section, contributing workflow, MIT license footer
  • Node.js prerequisite set to 18.18+ to match Next.js 16 requirements

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