Add hough_circles change to API doc

doc/source/api_changes.txt

@@ -3,6 +3,8 @@ Version 0.9
 - No longer wrap ``imread`` output in an ``Image`` class
 - Change default value of `sigma` parameter in ``skimage.segmentation.slic``
   to 0
+- ``hough_circle`` now returns a stack of arrays that are the same size as the
+  input image. Set the ``full_output`` flag to True for the old behavior.
 
 Version 0.4
 -----------