Add information to User Manual page about user docs for legacy vs fle…

…xible frameworks.

docs/sphinx/source/users-manual/index.rst

@@ -6,10 +6,51 @@
 
    .. image:: ../MATPOWER-md.png
 
-.. toctree::
-   :maxdepth: 2
-   :numbered: 3
-
 .. note::
 
    The new web-based version of the User's Manual is not yet available. Please, continue to use the PDF version of the |MUM| for now.
+
+
+Legacy Framework
+================
+
+.. note::
+
+   User documentation for the **legacy framework** is found in the legacy |MUM|.
+
+The legacy |MUM| decribes the **legacy** |MATPOWER| framework and interface, that is, the functionality and features fully compatible with |MATPOWER| 7.x or earlier.
+
+For example, you can run an AC OPF on the nine bus case as follows.
+
+.. code-block::
+
+   runopf('case9')
+
+By default, this uses the **legacy** |MATPOWER| framework, for full compatibility with legacy features and extensions, with MP-Core providing the underlying modeling. However, MP-Core can be bypassed completely, with legacy code being used for everything by passing an option to a specific run.
+
+.. code-block::
+
+   mpopt = mpoption('exp.use_legacy_core', 1);
+   runopf('case9', mpopt)
+
+Or MP-Core can be disabled globally for the current session with the following command, in which case no legacy commands will use MP-Core internally.
+
+.. code-block::
+
+   have_feature('mp_core', 0)
+
+
+New Flexible Framework
+======================
+
+.. note::
+
+   User documentation for the new **flexible framework** is found mainly in the |MATPOWER-Ref-Manual|, with some additional information in the |MATPOWER-Dev-Manual|.
+
+To run an AC OPF on the nine bus case in this mode you need to use the version of the ``run`` commands with an underscore in the name.
+
+.. code-block::
+
+   run_opf('case9')
+
+See the :ref:`ref_top_level_functions` section in the |MATPOWER-Ref-Manual| to get started.