added docs for the usage of column aliases

manual/codes/Getting started - Setting table definition - Column aliases.php

+<?php
+class Book extends Doctrine_Record 
+{
+    public function setTableDefinition() 
+    {
+        $this->hasColumn('bookName as name', 'string');
+    }
+}
+$book = new Book();
+$book->name = 'Some book';
+$book->save();
+?>

manual/docs/Getting started - Setting table definition - Column aliases.php

+Doctrine offers a way of setting column aliases. This can be very useful when you want to keep the application logic separate from the
+database logic. For example if you want to change the name of the database field all you need to change at your application is the column definition.