Update readme

README.md

@@ -51,12 +51,13 @@ end
 
 Often one finds the need to implement a stateful controller, i.e., a function
 that has a memory or state. To this end, the function [`sysfilter`](@ref) is
-provided. Usage is demonstrated below
+provided. This function is used to implement control loops where a signal is filtered through a
+dynamical system, i.e., `U(z) = C(z)E(z)`. Usage is demonstrated below
 ```julia
 stateC = init_sysfilter(C)
 function control(i)
     e  = r[i]-y[i]
-    ui = sysfilter!(stateC, C, e)
+    u  = sysfilter!(stateC, C, e)
 end
 ```
 `C` must here be represented by a [`StateSpace`](@ref) type from [`ControlSystems.jl`](https://github.com/JuliaControl/ControlSystems.jl).