Added more info to force documentation

README.md

@@ -103,7 +103,8 @@ functional should derive from this class, but the child type should then be used
 order to avoid the vtable penalty. The type `drum_t` is a `Drum` with the given
 template arguments. Typically, this functional would make heavy use of the `Drum`
 access members `get_parameters()` and `get_variables()`. The `time` argument of the
-functional exists to fit special cases as well.
+functional exists to fit special cases as well. The file `include/force_simple.hpp`
+showcases how a real force functional could be written.
 
 ### Template Type `Rk4Stepper` (`rk4_stepper.hpp`), performs timesteps using rk4 scheme
 1. Template arguments