Add usage example for builders to docs

doc/3_Code_generation.markdown

@@ -0,0 +1,59 @@
+Code generation
+===============
+
+...
+
+Builders
+--------
+
+The project provides builders for classes, methods, functions, parameters and properties, which
+allow creating node trees with a fluid interface, instead of instantiating all nodes manually.
+
+Here is an example:
+
+```php
+<?php
+$factory = new PHPParser_BuilderFactory;
+$node = $factory->class('SomeClass')
+    ->extend('SomeOtherClass')
+    ->implement('A\Few', 'Interfaces')
+    ->makeAbstract() // ->makeFinal()
+
+    ->addStmt($factory->method('someMethod')
+        ->makeAbstract() // ->makeFinal()
+        ->addParam($factory->param('someParam')->setTypeHint('SomeClass'))
+    )
+
+    ->addStmt($factory->method('anotherMethod')
+        ->makeProtected() // ->makePublic() [default], ->makePrivate()
+        ->addParam($factory->param('someParam')->setDefault('test'))
+        // it is possible to add manually created nodes
+        ->addStmt(new PHPParser_Node_Expr_Print(new PHPParser_Node_Expr_Variable('someParam')))
+    )
+
+    // properties will be correctly reordered above the methods
+    ->addStmt($factory->property('someProperty')->makeProtected())
+    ->addStmt($factory->property('anotherProperty')->makePrivate()->setDefault(array(1, 2, 3)))
+
+    ->getNode()
+;
+
+$stmts = array($node);
+echo $prettyPrinter->prettyPrint($stmts);
+```
+
+This will produce the following output with the default pretty printer:
+
+```php
+<?php
+abstract class SomeClass extends SomeOtherClass implements A\Few, Interfaces
+{
+    protected $someProperty;
+    private $anotherProperty = array(1, 2, 3);
+    abstract function someMethod(SomeClass $someParam);
+    protected function anotherMethod($someParam = 'test')
+    {
+        print $someParam;
+    }
+}
+```