[handbook] Add comment about handbook to doxygen process

doc/handbook/0_dumux-handbook.tex

@@ -115,6 +115,8 @@ Universit\"at Stuttgart, Paffenwaldring 61, D-70569 Stuttgart, Germany}\\
 \newpage
 
 \chapter{Overview and Infrastructure}
+The handbook is in the process of being dissolved.
+
 \input{5_structure}
 \input{5_newfoldersetup}
 \input{5_scripts}