Added project_to_viewport to exporters docs

docs/import_export.rst

@@ -108,6 +108,39 @@ specific properties followed by the addition of layers and shapes to the documen
 Once all of the layers and shapes have been added, the document can be written
 to a file. 
 
+3D to 2D Projection
+-------------------
+
+There are a couple ways to generate a 2D drawing of a 3D part:
+
+* Generate a section: The  :func:`~operations_part.section` operation can be used to
+  create a 2D cross section of a 3D part at a given plane.
+* Generate a projection: The :meth:`~topology.Shape.project_to_viewport` method can be
+  used to create a 2D projection of a 3D scene. Similar to a camera, the ``viewport_origin``
+  defines the location of camera, the ``viewport_up`` defines the orientation of the camera,
+  and the ``look_at`` parameter defined where the camera is pointed.  By default, 
+  ``viewport_up`` is the positive z axis and ``look_up`` is the center of the shape.  The
+  return value is a tuple of lists of edges, the first the visible edges and the second
+  the hidden edges.  
+  
+Each of these Edges and Faces can be assigned different line color/types and fill colors
+as described below (as ``project_to_viewport`` only generates Edges, fill doesn't apply).  
+The shapes generated from the above steps are to be added as shapes 
+in one of the exporters described below and written as either a DXF or SVG file as shown
+in this example:
+
+.. code-block:: python
+
+    view_port_origin=(-100, -50, 30)
+    visible, hidden = part.project_to_viewport(view_port_origin)
+    max_dimension = max(*Compound(children=visible + hidden).bounding_box().size)
+    exporter = ExportSVG(scale=100 / max_dimension)
+    exporter.add_layer("Visible")
+    exporter.add_layer("Hidden", line_color=(99, 99, 99), line_type=LineType.ISO_DOT)
+    exporter.add_shape(visible, layer="Visible")
+    exporter.add_shape(hidden, layer="Hidden")
+    exporter.write("part_projection.svg")
+
 LineType
 --------