updated release notes

Author: wesm

RELEASE.rst

@@ -100,6 +100,13 @@ Release notes
 
 * The `DataMatrix` variable now refers to `DataFrame`, will be removed within
   two releases
+* Cython is now required to build `pandas` from a development branch. This was
+  done to avoid continuing to check in cythonized C files into source
+  control. Builds from released source distributions will not require Cython
+* Cython extension modules have been renamed and promoted from the `lib`
+  subpackage to the top level, i.e.
+  * `pandas.lib.tseries` -> `pandas._tseries`
+  * `pandas.lib.sparse` -> `pandas._sparse`
 * A `copy` argument has been added to the `DataFrame` constructor to avoid
   unnecessary copying of data. Data is no longer copied by default when passed
   into the constructor