added a 'validate' option to Document.save() +docs +tests

docs/guide/defining-documents.rst

@@ -214,6 +214,20 @@ either a single field name, or a list or tuple of field names::
         first_name = StringField()
         last_name = StringField(unique_with='first_name')
 
+Skipping Document validation on save
+------------------------------------
+You can also skip the whole document validation process by setting 
+``validate=False`` when caling the :meth:`~mongoengine.document.Document.save` 
+method::
+
+    class Recipient(Document):
+        name = StringField()
+        email = EmailField()
+    
+    recipient = Recipient(name='admin', email='root@localhost')
+    recipient.save()               # will raise a ValidationError while
+    recipient.save(validate=False) # won't
+
 Document collections
 ====================
 Document classes that inherit **directly** from :class:`~mongoengine.Document`