Update README with missing feature descriptions

The README only had a one-line description. Added sections covering:
- Version-aware stubs with #[\Since] and #[\Until] attributes
- Coverage scope (core, 69 extensions, 6 SAPIs)
- Php8StubsMap version-aware class/function mapping
- Automated extraction and daily CI pipeline
- Basic usage example
- Dual license information

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: 2 people

README.md

@@ -1,4 +1,24 @@
 PHP 8 stubs
 ====================
 
-The stubs are automatically extracted from [php-src](https://github.com/php/php-src/) and are relevant for analyzing codebases that use PHP 8 or later.
+PHP stub files automatically extracted from [php-src](https://github.com/php/php-src/). Used by [PHPStan](https://phpstan.org/) to understand signatures of built-in PHP classes and functions for PHP 8.0+.
+
+## Features
+
+- **Version-aware stubs** covering PHP 8.0 through 8.5, with `#[\Since('x.y')]` and `#[\Until('x.y')]` attributes to track when symbols and signatures were added or removed.
+- **Comprehensive coverage**: core language stubs (`Zend/`), 69 extensions (`ext/`), and 6 SAPIs (`sapi/`).
+- **Version-aware class/function map** (`Php8StubsMap`) that accepts a `$phpVersionId` and returns only the symbols available in that PHP version.
+- **Fully automated extraction** via `extractor/extract.php` — stubs are never hand-written.
+- **Daily CI updates** that extract from all PHP 8.x branches, auto-tag releases, and open PRs in `phpstan/phpstan-src`.
+
+## Usage
+
+```php
+$map = new \PHPStan\Php8StubsMap($phpVersionId); // e.g. 80300 for PHP 8.3.0
+$map->classes;   // array<string, string> — lowercase class name => stub path
+$map->functions; // array<string, string> — lowercase function name => stub path
+```
+
+## License
+
+Dual-licensed under [MIT](LICENSE) and [PHP-3.01](stubs/LICENSE).