Improve documentation for HTTPlug HTTP client requirement

[Ref34t]

Background

The WordPress PHP AI Client correctly uses HTTPlug for HTTP client abstraction and auto-discovery. However, new users aren't aware they need to install an HTTPlug-compatible HTTP client adapter.

Current Experience

1. User installs: composer require wordpress/php-ai-client
2. User runs code and gets: DiscoveryFailedException: Could not find resource...
3. User must research HTTPlug and PSR-18 to understand the solution

Proposed Documentation Improvements

Add to README.md:

HTTP Client Requirement

This package uses HTTPlug for HTTP client abstraction. You need to install a compatible HTTP client adapter:

# Choose one HTTP client adapter:
composer require php-http/guzzle7-adapter     # Guzzle 7 (recommended)
composer require php-http/curl-client         # cURL
composer require symfony/http-client          # Symfony

Add composer.json suggestions:

"suggest": {
  "php-http/guzzle7-adapter": "HTTPlug adapter for Guzzle 7 (recommended)",
  "php-http/curl-client": "HTTPlug adapter for cURL"
}

This maintains the correct HTTPlug architecture while improving the developer experience.

[Ref34t]

@felixarntz, I've encountered this situation while I was trying to use the package, also, why don't we just add the HTTP client adapter to the required dependencies?