doc: driver-model: Convert README.txt to reST

Convert plain text documentation to reStructuredText format and add
it to Sphinx TOC tree. No essential content change.
Signed-off-by: NBin Meng <bmeng.cn@gmail.com>
Reviewed-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>

doc/driver-model/index.rst

@@ -5,3 +5,5 @@ Driver Model
 
 .. toctree::
    :maxdepth: 2
+
+   design