added the LeafElementIterator-issue

[[Imported from SVN: r244]]

KnownIssues

@@ -10,9 +10,19 @@ The following issues are known when using dune-subgrid:
     <li>
         Currently only grids with elements of a single geometry work as hostgrids.
     </li>
-    <li> 
+    <li>
         LeafIntersectionIterator can return wrong intersections if working with dune-grid1.3svn Revision &lt; 5101  
     </li>
+    <li>
+        creating the (leaf-) end iterator can be costly (especially with ALU3dSimplexGrid). For a walk over all 
+        (leaf-) entities we strongly(!) recommend creating the end iterator outside the loop:
+        Example:
+        <code>
+            ElementLeafIterator end=grid.leafend<0>();
+            for (ElementLeafIterator eltIt=grid.leafbegin<0>(); eltIt!=end; ++eltIt)
+            {...}
+        </code>
+    </li>
 </ul>
 Subgrid uses certain host grid features where you might not expect it.
 If a subgrid behaves strange this might be caused by bugs in the host grid