[handbook] cleanup

doc/handbook/3_furtherpractice.tex

@@ -14,6 +14,9 @@ The \texttt{dumux-lecture} module can be obtained as follows:
 \begin{lstlisting}[style=Bash]
 $ git clone https://git.iws.uni-stuttgart.de/dumux-repositories/dumux-lecture.git
 \end{lstlisting}
+% TODO: remove again as soon as invalid:
+Note that \texttt{dumux-lecture} is currently only compatible with the \Dumux release 2.12.
+
 The module is structured based on the different lectures: 
 \begin{itemize}
 \item mm: Multiphase Modelling,