Added the 'CR72 red' model to the relax user manual.
This is the Carver and Richards 1972 analytic model with the simplification R20A = R20B. The
current 'CR72' makes the same assumption, but that model will be expanded to support R20A and R20B
later.
This commit follows step 2 of the relaxation dispersion model addition tutorial
(http://thread.gmane.org/gmane.science.nmr.relax.devel/3907).
