README Rewrite: Positioning & Limitations

[muditbhargava66]

Goal: Sharpen the README to clearly position the simulator.

Actions:

• Answer:
  • What this simulator is best at?
  • What it intentionally does NOT model?
  • When to use this instead of gem5?
  • When NOT to use it?
• Add a "Limitations" section explicitly.

Positioning: A modular, trace-driven cache hierarchy simulator for rapid exploration, validated against established tools.