Add comprehensive custom integration guide to README

Document all helpers and best practices learned from production
integration into a real SaaS application.

New sections:
- Building Custom Integrations (overview + what you'll need)
- Quick Setup (4-step guide with code)
- Complete Controller Example (90-line production-ready code)
- Model Scopes (.publishable, .secret, .active, etc.)
- Owner Instance Methods (available_api_key_scopes, can_create_api_key?, etc.)
- API Key Instance Methods (full method reference)
- Token Session Helper (store, retrieve_once, available?)
- Expiration Options Helper (for_select, parse, default_value)
- Form Builder Extensions (with ERB examples)
- View Helpers (status, type, environment helpers)
- View Examples (index, form, success page code)
- Best Practices (7 production-learned patterns)

The guide provides copy-paste-ready code for building custom UIs
while using the gem's model layer and business logic.

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

Author: rameerez