Add a bit of documentation

design/parser.md

+These notes explain how to load a model and render it as a statachart
+
+This can be done for example using ptpython3
+
+
+    # Import the opengeode library to access the parser
+    import opengeode
+
+    # Make sure Qt is initialized to render the model
+    app = opengeode.init_qt()
+
+    # Parse a model
+    ast, _, _ = opengeode.ogParser.parse_pr(['model.pr'])
+
+    # ast is of type opengeode.ogAST.AST (check ogAST.py)
+
+    # Extract the SDL process
+    process = ast.processes[0]  # type: opengeode.ogAST.Process
+
+    # Create a scene and a view to render the process as a statechart
+    scene = opengeode.SDL_Scene('statechart')
+    view = opengeode.SDL_View(scene)
+
+    # Transform the AST of the process into a graphviz graph
+    graph = opengeode.Statechart.create_dot_graph(process, basic=True)
+  
+    # Then render the graph (requires graphviz)
+    opengeode.Statechart.render_statechart(scene, graph)
+
+    # Enable the view and run the application
+    view.show()
+    app.exec_()
+
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