[doc] add removal of entity pointer to change log

This also documents the the dimensions are gone from entity iterator,
closing #59.

CHANGELOG.md

 # master (will become 2.6)
 
+- The deprecated `EntityPointer` has been removed completely and `EntityIterator`
+  no longer inherits from it.
+  As a consequence, the dimension `EntityIterator::dimension`,
+  `EntityIterator::codimension`, and `EntityIterator::mydimension` are gone.
 - Experimental grid extensions are now always enabled.
 - The method `impl` and the type `Implementation` on the facade classes are
   always public (and documented), now.