Add comprehensive project README

Document the Java Swing + MySQL course management system: feature
breakdown per role, architecture overview, tech stack, project layout,
full database schema, and build/run instructions for Windows and Unix.

Also records the findings of a security review (SQL injection in all
login and data screens, plaintext password storage, missing session
model, outdated Connector/J) alongside known functional defects, so the
risks are visible rather than implicit.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: Alphax978