Updating the documentation

[phajy]

Once the code has been tidied up we'll need to update the documentation. This is currently not compiling but the code tidy will fix that. Some things I think we need to do for the documentation:

• Update the README with a simple example of how to load the module and do a simple fit.
• Ensure there is in-line documentation for the relevant exported functions.
• Have more extensive documentation on how the model can be set up and fit.
• Make sure the documentation compiles automatically.