[handbook] Add link to DUNE cheatsheet.

doc/handbook/1_introduction.tex

@@ -49,7 +49,10 @@ entity can be thought of as a prototype for the actual grid
 entity. For example, if we used a grid which applied hexahedrons as cells,
 the reference element for each cell would be the unit cube $[0, 1]^3$
 and the geometry function would scale and translate the cube so that
-it matches the grid's cell. For a more thorough description of \Dune's
+it matches the grid's cell. A quick overview of reference elements and the
+related numbering can be gotten from the DUNE cheat sheet
+(\url{https://www.dune-project.org/pdf/dune-cheat-sheet.pdf}).
+For a more thorough description of \Dune's
 grid definition, see~\cite{BASTIAN2008}.
 
 In addition to the grid interface, \Dune also provides quite a few