Merge pull request numpy#12148 from shoyer/nep18-release-notes

DOC: document NEP-18 overrides in release notes

doc/release/1.16.0-notes.rst

@@ -227,4 +227,14 @@ c-extension module.
 `numpy.ndarray.getfield` now checks the dtype and offset arguments to prevent
 accessing invalid memory locations.
 
+NumPy functions now support overrides with ``__array_function__``
+-----------------------------------------------------------------
+It is now possible to override the implementation of almost all NumPy functions
+on non-NumPy arrays by defining a ``__array_function__`` method, as described
+in `NEP 18`_. The sole exception are functions for explicitly casting to NumPy
+arrays such as ``np.array``. As noted in the NEP, this feature remains
+experimental and the details of how to implement such overrides may change in
+the future.
+
 .. _`NEP 15` : http://www.numpy.org/neps/nep-0015-merge-multiarray-umath.html
+.. _`NEP 18` : http://www.numpy.org/neps/nep-0018-array-function-protocol.html