update CHANGELOG

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@13586 2fb0f335-1f38-0410-981e-8018bf24f1b0

CHANGELOG

+Differences Between DuMuX 2.6 and DuMuX 2.7
+===================================================
+
+* DELETED classes/files, property names, constants/enums, 
+  member functions, which have been deprecated in DuMuX 2.6:
+  Everything listed as deprecated below has been removed.
+
+
 Differences Between DuMuX 2.5 and DuMuX 2.6
 ===================================================