Added link to the documentation page about the model shell structure.

docs/physics/tardisgamma/packetsampling.rst

@@ -2,7 +2,7 @@ Packet Sampling
 ===============
 
 
-In the simulation, we use spherical symmetry where the ejecta is divided into multiple spherical shells. To find the mass of each shell we multiply the mass fractions of each of the elements and isotopes with the density. We can set the timespace for the simulation
+In the simulation, we use spherical symmetry where the ejecta is divided into multiple spherical shells(see :doc:`../setup/model`). To find the mass of each shell we multiply the mass fractions of each of the elements and isotopes with the density. We can set the timespace for the simulation
 to be between t\ :sub:`start`\  and t\ :sub:`end`\  using either a linear or a logarithmic scale.