Document API changes

doc/source/api_changes.txt

@@ -4,6 +4,14 @@ Version 0.12
   the ``ntiles_*`` arguments.
 - The functions ``blob_dog``, ``blob_log`` and ``blob_doh`` now return float
   arrays instead of integer arrays.
+- `reverse_map` parameter in `skimage.transform.warp` has been removed.
+- `enforce_connectivity` in `skimage.segmentation.slic` defaults to ``True``.
+- `skimage.measure.fit.BaseModel._params`,
+  `skimage.transform.ProjectiveTransform._matrix`,
+  `skimage.transform.PolynomialTransform._params`,
+  `skimage.transform.PiecewiseAffineTransform.affines_*` attributes
+  have been removed.
+- `skimage.filters.denoise_*` have moved to `skimage.restoration.denoise_*`.
 
 Version 0.11
 ------------