Add warning to limiter section.

12_coupling-limiter.tex

@@ -254,6 +254,10 @@ with each cell being divided in a set of $2N+1$ subcells. Taken from~\cite{ADERC
 \end{figure}
 
 \section{Optional tuning parameters}
+\begin{warning}
+This section is not up to date anymore.
+\end{warning}
+
 The \texttt{Limiting-ADER-DG} solver offers an optional tuning parameter
 \texttt{helper-layers}. This parameter can be used for
 enabling the solver to anticipate the propagation of troubled cells.