docs: add Getting Started section and contribution table (closes #59)

[zqleslie]

Summary

Addresses issue #59 for Assignment A14.

Changes

Added a comprehensive "Getting Started" section and "Contribution Roadmap" table to README.md:

Getting Started

• Prerequisites: Python 3.10+, Git, pip
• Setup instructions: clone, install, run tests, start API (4-step guide with code blocks)
• Project Structure table: directory → description mapping

Contribution Roadmap

• Feature table with label, difficulty, and linked issue columns
• Links to CONTRIBUTING.md and ROADMAP.md

Header

• CI status badge: [![CI](https://github.com/Anamandla/SwiftPay/actions/workflows/ci.yml/badge.svg)]

Acceptance Criteria

• "Getting Started" section: prerequisites, clone, run tests, start API
• "Features for Contribution" table: feature name, label, difficulty, linked issue
• CI status badge added to README header
• Links to CONTRIBUTING.md and ROADMAP.md prominent in README

Closes #59

[mayank-dev-15]

Good docs - Getting Started section and contribution table improve onboarding. LGTM.