Add a comment about the include guard

[[Imported from SVN: r12761]]

dune/solvers/common/interval.hh

 #ifndef DUNE_SOLVERS_COMMON_INTERVAL_HH
 #define DUNE_SOLVERS_COMMON_INTERVAL_HH
 
+// Note: This file exists both in dune-solvers and in dune-fufem. To
+// avoid redefinitions, please make sure that they have *identical*
+// include guards.
+
 #include <iostream>
 #include <algorithm>