Added example with config and motion constraints.

[moorepants]

Example

• There are no merge conflicts.
• A figure (preferrably SVG) has been added for the example.
• The required versions of all dependencie have been noted. (For
  notebooks, the version information
  extension is helpful
  for this.)
• A run.py file that executes the example is included.
• All reviewer comments have been addressed.

[moorepants]

I'm not going to add a figure. I think the text is sufficient. This works with the current pydy dependencies.