docs: add README with Features section and quick-start guide

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -0,0 +1,23 @@
+# OSDB — Obstsortendatenbank
+
+A full-stack fruit-variety database (Go + Vue 3).
+
+## Features
+
+- Users can view a "Hello, OSDB!" landing page that confirms the backend is reachable via the Vite proxy.
+
+## Quick Start
+
+```bash
+cp .env.example .env          # set credentials
+make dev-db                   # start Postgres (waits until healthy)
+make migrate-up               # apply schema migrations
+make run-backend              # Echo API on :3000
+make run-frontend             # Vite dev server on :5000
+```
+
+## Testing
+
+```bash
+make test
+```