Add detailed information about:
- Project overview and key features
- Complete source code structure (Lexer, Parser, AST, Printer)
- Test organization and structure
- How the parser works (Lexing → Parsing → Traversal → Printing)
- Common development tasks (adding tags, types, modifying lexer)
- Coding standards and best practices
- PHPStan rules and testing guidelines
- AST node conventions and parser patterns
- Backwards compatibility considerations
- Performance and documentation guidelines

Add __set_state() magic method to all 68 AST node classes to enable
var_export() serialization for caching purposes.

Implementation details:
- All classes now support reconstruction via var_export()
- Constructor parameters are properly extracted from properties array
- Attributes are restored after instance creation
- Optional parameters handled with null coalescing operator

Classes updated:
- Type nodes: 20 classes (ArrayTypeNode, GenericTypeNode, etc.)
- ConstExpr nodes: 10 classes (ConstExprArrayNode, ConstFetchNode, etc.)
- PhpDoc core: 4 classes (PhpDocNode, PhpDocTagNode, etc.)
- PhpDoc tag values: 29 classes (ParamTagValueNode, ReturnTagValueNode, etc.)
- Doctrine nodes: 5 classes (DoctrineAnnotation, DoctrineArray, etc.)

All tests pass (3711 tests, 19858 assertions)
PHPStan analysis passes with no errors