[handbook] Introduce to 3.0 and add link to wiki

Ideas for a better last sentence are certainly welcome :)

doc/handbook/1_introduction.tex

@@ -74,3 +74,17 @@ spatial and temporal discretization schemes as well as nonlinear solvers,
 to general concepts for model coupling.
 Moreover, \Dumux includes ready to use numerical models and a few example applications.
 
+This is the handbook to a new major version update of \Dumux: version 3.0.
+The release contains considerable improvements and many new features compared to the 2.x versions. 
+Due to the major update, backwards compatibility with the last release 2.12 cannot be assured.
+To facilitate the transition for our users, we have created a
+git wiki entry describing how to update programs from version 2.12 to version 3.0.
+It is available online:
+\url{https://git.iws.uni-stuttgart.de/dumux-repositories/dumux/wikis/Updating-programs-from-version-2.12-to-version-3.0}.
+The guide leads in detail through the interface changes from 2.12 to 3.0,
+concerning the \texttt{Problem} class, the \texttt{SpatialParams} class,
+the \texttt{Parameters} and \texttt{Properties}, i.e. the main user interface.
+Starting with version 3.0.0, all minor
+version updates will certainly be backward compatible again with at least the last minor version.
+We highly recommend all our users to transition with us to \Dumux-3.0
+and wish everyone a brand-new and exciting simulation experience.