Added the 'NS 2-site star red' model to the relax user manual.
This is the model of the numerical solution for the 2-site Bloch-McConnell equations using complex
conjugate matrices, whereby the simplification R20A = R20B is assumed.
This commit follows step 2 of the relaxation dispersion model addition tutorial
(http://thread.gmane.org/gmane.science.nmr.relax.devel/3907).
