DOC: Add descriptions for each example model

[nawtrey]

There is a README in every example directory but they are currently empty. It would be nice if these were populated with a description of the model.

Examples to update

• 3-state model
• 4-state model
• 4-state model with leakage
• 5-state model with leakage
• 6-state model
• 6-state model with leakage
• 8-state model with leakage