added docs for notnull constraint

manual/docs/Object relational mapping - Constraints and validators - Notnull.php

+A not-null constraint simply specifies that a column must not assume the null value. A not-null constraint is always written as a column constraint.
+
+The following definition uses a notnull constraint for column 'name'. This means that the specified column doesn't accept
+null values.
+
+<code type='php'>
+class User extends Doctrine_Record
+{
+    public function setTableDefinition()
+    {
+        $this->hasColumn('name', 'string', 200, array('notnull' => true,
+                                                      'primary' => true));
+    }
+}
+</code>
+
+When this class gets exported to database the following Sql statement would get executed (in Mysql):
+
+CREATE TABLE user (name VARCHAR(200) NOT NULL, PRIMARY KEY(name))