Added the 'MQ NS CPMG 2-site' model to the relax user manual.

This is the 2-site numeric solution for multi-quantum CPMG-type data.

This follows the tutorial for adding relaxation dispersion models at:
http://wiki.nmr-relax.com/Tutorial_for_adding_relaxation_dispersion_models_to_relax#The_relax_manual.