add whatsnew entry

doc/source/pre/whatsnew/1.5.rst

@@ -4,8 +4,8 @@ What's new in Kwant 1.5
 This article explains the user-visible changes in Kwant 1.5.0.
 
 
-Value functions may now be vectorized
--------------------------------------
+Kwant supports vectorized value functions
+-----------------------------------------
 It is now possible to define value functions that act on whole
 arrays of sites at a time.
 ::
@@ -36,6 +36,19 @@ order of magnitude speedup when evaluating system Hamiltonians (though this
 speedup may be masked by other parts of your computation e.g. calculating
 the scattering matrix).
 
+Similarly, the ``onsite`` passed to an operator may now be vectorized in the same
+way, as long as the system passed to the operator is also vectorized:
+::
+
+    def x_onsite(sites):
+        x, _ = sites.positions().transpose()
+        return x
+
+    x_position = kwant.operator.Density(syst, x_onsite)
+
+    psi = kwant.wave_function(syst, energy=0.1)(0)[0]
+    x_position(psi)
+
 
 Deprecation of leaving 'norbs' unset for site families
 ------------------------------------------------------