Add reference to the TBMT model

docs/source/run/parameters.rst

@@ -1044,7 +1044,10 @@ Spin tracking
 -------------
 
 Track the spin of each beam particle as it is rotated by the electromagnetic fields using the
-TBMT model. This will add three extra components to each beam particle to store the spin and output
+Thomas-Bargmann-Michel-Telegdi (TBMT) model, see
+[Z. Gong et al., Matter and Radiation at Extremes 8.6 (2023), https://doi.org/10.1063/5.0152382]
+for the details of the implementation.
+This will add three extra components to each beam particle to store the spin and output
 those as part of the beam diagnostic.
 
 * ``<beam name> or beams.do_spin_tracking`` (`bool`) optional (default `0`)