Update README.md

README.md

@@ -64,6 +64,22 @@ Since the dune-grid StructuredGridFactory does not allow to pass an own
 GridFactory object to fill, a modified StructuredGridFactory is provided in the
 utility folder, called `StructuredGridBuilder`.
 
+Grid iteration
+--------------
+
+In order to iterate over the combined grid, the MultiMesh traverses all elements
+hierarchically, starting from the coarse-grid. If an entity reaches its leaf-level,
+it is conserved until the entities from the other grids also reach its leaf-level.
+Then a vector of entities is created and returned by the LeafIterator:
+
+```c++
+for (auto const& multiEntity : elements(grid.leafGridView())) {
+  assert(multiEntity.size() == grid.size());
+  for (auto const& entity : multiEntity)
+    assert(entity.isLeaf());
+}
+```
+
 TODO
 ----
 The current implementation is very limited. Several things need to be