docs: inline SDK comments, integration guide, README, license

[kherembourg]

Summary

• // PURCHASELY: inline comments on every SDK call across both Android (9 files, 54 lines) and iOS (10 files, 62 lines), with docs links
• Companion Integration Guide (docs/INTEGRATION_GUIDE.md) — 10-section walkthrough of every SDK feature with dual-platform code snippets, console setup steps, and common pitfalls
• Marketing README — feature checklist, quick start, architecture overview, placements table
• MIT License (copyright Purchasely 2026)
• Contributing Guide — setup, conventions, how to add new SDK features

Documentation structure

README.md              — Public-facing entry point
LICENSE                — MIT
CONTRIBUTING.md        — Developer onboarding
docs/INTEGRATION_GUIDE.md — Feature-by-feature SDK reference

Test plan

• Verify inline comments don't break builds (both platforms)
• Review INTEGRATION_GUIDE.md code snippets match actual app code
• Review README quick start instructions work on a fresh clone

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