Add some more documentation

src/03-function-integration.cc

@@ -41,6 +41,7 @@ double integrate(const GridView& gridView, const F& f, std::size_t order)
 
     const auto& quad = Dune::template QuadratureRules<double, dim>::rule(geometryType, order);
 
+    // Loop over all quadrature points
     for (size_t i=0; i < quad.size(); ++i)
     {
       const auto& quadPos = quad[i].position();