Added capped collections support

docs/userguide.rst

@@ -153,6 +153,21 @@ document class to use::
         title = StringField(max_length=200, required=True)
         meta = {'collection': 'cmsPage'}
 
+Capped collections
+^^^^^^^^^^^^^^^^^^
+A :class:`~mongoengine.Document` may use a **Capped Collection** by specifying
+:attr:`max_documents` and :attr:`max_size` in the :attr:`meta` dictionary.
+:attr:`max_documents` is the maximum number of documents that is allowed to be
+stored in the collection, and :attr:`max_size` is the maximum size of the
+collection in bytes. If :attr:`max_size` is not specified and
+:attr:`max_documents` is, :attr:`max_size` defaults to 10000000 bytes (10MB).
+The following example shows a :class:`Log` document that will be limited to 
+1000 entries and 2MB of disk space::
+
+    class Log(Document):
+        ip_address = StringField()
+        meta = {'max_documents': 1000, 'max_size': 2000000}
+
 Document inheritance
 --------------------
 To create a specialised type of a :class:`~mongoengine.Document` you have