Write docs on creating a single object

src/blenderbim/docs/users/creating_an_ifc_model.rst

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+Creating an IFC model
+=====================
+
+We're going to create a new native IFC model from scratch, starting from a
+single shape, up to a simple building.
+
+A BIM model is not just a 3D format. This means that you cannot just load any 3D
+model, and press a "Save as IFC" button. In fact, 3D is optional and you
+can have a large IFC model storing work schedules, cost rates, asset registers
+and more, and absolutely no 3D geometry at all.
+
+However, in these tutorials, we will focus on basic 3D geometry. At a minimum,
+you will need to categorise your 3D elements (such as "Wall", "Column",
+"Window", etc), and give them a location (such as "Level 01" of a building).
+
+Creating a single object
+------------------------
+
+In the **Properties** panel in the bottom right, activate the icon for the
+**Scene Properties** tab, and find the **IFC Project** subpanel. Click on
+**Create Project** to create a blank IFC project.
+
+.. image:: create-project.png
+
+For convenience, this IFC project by default defines length, area, and volume
+units, and assumes you'll be working with both 2D and 3D geoemtry.
+
+In the top right **Outliner** panel, you will see a hierarchy of spaces that has
+been automatically created for you.  This hierarchy is known is the **Spatial
+Tree**.
+
+.. image:: default-spatial-tree.png
+
+Any Blender object that you want to part of your IFC project must be converted
+into a IFC object by assigning a category. This category is known as the **IFC
+Class**.
+
+Select only the default Blender Cube (selected objects are highlighted in
+orange, careful not to select anything else!), switch to the **Object
+Properties** tab, and find the **IFC Class** panel. Let's pretend our Cube is a
+column, so select **IfcColumn** from the **Class** drop-down, and press **Assign
+IFC Class**.
+
+.. image:: assign-class.png
+
+All IFC objects must also belong inside the **Spatial Tree**. In the
+**Outliner** panel, you can drag and drop your newly created **IfcColumn/Cube**
+inside the **IfcBuildingStorey/Ground Floor**. Wasn't that simple? 
+
+.. image:: drag-drop-outliner-tree.png
+
+Go back to the **Scene Properties** tab and to the **IFC Project** panel and
+press **Save Project**. Save your new IFC model somewhere on your computer.
+
+.. image:: save-project.png
+
+Congratulations! You have now created your first OpenBIM model from Blender! You
+can open the IFC file in any other program, and you will see something similar
+to the image below. Three simple open source online viewers you can test with
+are `IfcPipeline <https://view.ifcopenshell.org>`__, `IFC.JS
+<https://ifcjs.github.io/web-ifc-viewer/example/index>`__, and `3DViewer
+<https://3dviewer.net/>`__.
+
+.. image:: ifc-pipeline.png
+
+Placing occurrences of an element type
+--------------------------------------
+
+TODO
+
+Changing the locations of elements
+----------------------------------
+
+TODO
+
+Modeling a simple building
+--------------------------
+
+TODO