Update README.rst.

README.rst

@@ -47,3 +47,39 @@ date as follows:
 :code:`XValidatedModel` ensures that this specification is maintained
 invoking :code:`Event.clean()` and raises :code:`ValidationError` as
 appropriate.
+
+Why use django-xvalidate?
+-------------------------
+django-xvalidate allows you to specify how to validate your model instances
+in a more declarative manner than writing imperative code within your
+:code:`clean()` methods. Without django-xvalidate you would have to
+implement the above example as
+
+.. code:: python
+
+    def clean(self):
+        super(Event, self).clean()
+        if (self.start_date is not None) and (self.end_date is not None):
+            if (self.end_date < self.start_date):
+                raise ValidationError('The start date should precede the end date')
+
+With a more declarative format we have the option at some point in the
+future to automate the creation of test data that passes (or fails)
+validation.
+
+django-xvalidate comes some operator overloading that brings syntactic
+sugar to your declarations making them very easy to read. For instance,
+you could specify:
+
+.. code:: python
+    ((XF('end_date') - 'start_date') > datetime.timedelta(days=4)).message(
+        'Event should last at least 5 days'
+    )
+
+django-xvalidate also allows the use of Django's double-underscore (`__`)
+syntax to dereference related objects, enabling succinct definitions such
+as the following
+
+.. code:: python
+    (XF('registration_date') <= 'event__end_date').message(
+        'Must register before the event ends')