add new option in the docs

Author: jhdark

docs/source/userguide/stepsize.rst

@@ -68,4 +68,15 @@ This will modify the stepsize as needed.
         dt_min=1e-6,
         max_stepsize=5,
         milestones=[1, 5, 6, 10]
-        )
+        )
+
+
+.. note::
+    
+    When defining the simulation, if your stepsize is constant, you can define it simply by providing it as a `float`` or `int`:
+    
+    .. testcode::
+
+        my_model = F.Simulation()
+
+        my_model.dt = 2.0