add basic concept of modelspace
Modelspace
==========
+The modelspace contains the "real" world representation of the drawing subjects
+in real world units and is displayed in the tab called "Model" in CAD
+applications. The modelspace is always present and can't be deleted.
+
+The modelspace object is acquired by the method :meth:`~ezdxf.document.Drawing.modelspace`
+of the :class:`~ezdxf.document.Drawing` class and new entities
+should be added to the modelspace by factory methods: :ref:`thematic_factory_method_index`.
+
+This is a common idiom to create new document and acquiring the modelspace::
+
+ import ezdxf
+
+ doc = ezdxf.new()
+ msp = doc.modelspace()
