doc: add estimation horizon in README.md

Author: franckgaga

README.md

@@ -51,14 +51,14 @@ Our goal is controlling the first output $y_1$, but the second one $y_2$ should
 35:
 
 ```julia
-mhe = MovingHorizonEstimator(model)
+mhe = MovingHorizonEstimator(model, He=10)
 mpc = LinMPC(mhe, Mwt=[1, 0], Nwt=[0.1])
 mpc = setconstraint!(mpc, ymax=[Inf, 35])
 ```
 
-The keyword arguments `Mwt` and `Nwt` are the output setpoint tracking and move suppression
-weights, respectively. A setpoint step change of five tests `mpc` controller in closed-loop.
-The result is displayed with [`Plots.jl`](https://github.com/JuliaPlots/Plots.jl):
+The keyword arguments `He`, `Mwt` and `Nwt` are the estimation horizon, the output setpoint
+tracking and move suppression weights, respectively. A setpoint step change of five tests
+`mpc` controller in closed-loop. The result is displayed with [`Plots.jl`](https://github.com/JuliaPlots/Plots.jl):
 
 ```julia
 using Plots