Enhance README with quick start guide and algorithm overview

[domforson]

Summary

Enhanced the README to improve the new user experience by adding getting-started information and project structure overview.

Problem

The existing README provided a high-level introduction to Spinning Up but did not have guidance for new users like myself (seemed to be written by experts, not anticipating newbie problems). As someone learning deep reinforcement learning, I found myself spending a lot of time trying to understand the codebase.

Changes Made

Quick Installation Section

• Added step-by-step installation commands
• Documented prerequisites (Python 3.6+, OpenMPI)
• platform support (Linux/macOS, Windows limitations)
• hyperlinked to installation guide for MuJoCo setup

Quick Start Guide

• Added example using PPO on LunarLander environment
• Included commands for training, testing, and plotting results
• Enables users to verify installation and see immediate results

Algorithm Overview

• Created table of all 6 implemented algorithms in this repo
• Specified framework availability (TensorFlow 1 and PyTorch)
• Categorized algorithms by type (On-Policy vs Off-Policy)
• Added hyperlinks to relevant code directories

Repository Structure Visualization

• Added Mermaid diagram showing project organization
• hierarchy from root to specific algorithm files