FindMatlab: add change log item.

Help/release/dev/FindMatlab-2018a-API.rst

@@ -0,0 +1,7 @@
+FindMatlab-2018a-API
+--------------------
+
+* The :module:`FindMatlab` module gained new options ``R2017b`` and
+  ``R2018a`` to specify the MEX API version to use; these options
+  mirror the new options to the ``mex`` command in MATLAB R2018a.
+  The option ``MX_LIBRARY`` is no longer needed.