Add autoupdate field to design docs API

Background indexing in CouchDB can be controlled on a
per-design document basis. This documents the option to
disable automatic indexing by specifying `autoupdate: false`
as a top level field in a design document.
diff --git a/src/api/ddoc/common.rst b/src/api/ddoc/common.rst
index 68da199..31673d7 100644
--- a/src/api/ddoc/common.rst
+++ b/src/api/ddoc/common.rst
@@ -57,6 +57,8 @@
     * **validate_doc_update** (*string*): :ref:`Validate document update
       <vdufun>` function source
     * **views** (*object*): :ref:`View functions <viewfun>` definition.
+    * **autoupdate** (*boolean*): Indicates whether to automatically build
+      indexes defined in this design document. Default is `true`.
 
     Note, that for ``filters``, ``lists``, ``shows`` and ``updates`` fields
     objects are mapping of function name to string function source code. For