adding whatsnew entry

docs/whatsnew/5.3.rst

@@ -19,6 +19,7 @@ In particular, this release includes:
 .. * :ref:`whatsnew-5.3-unit-formats-fraction`
 .. * :ref:`whatsnew-5.3-unit-formats-order`
 .. * :ref:`whatsnew-5.3-nddata-collapse-arbitrary-axes`
+.. * :ref:`_whatsnew-5.3-modeling-measurement-uncertainties`
 .. * :ref:`whatsnew-5.3-coordinates-refresh-site-registry`
 
 In addition to these major changes, Astropy v5.3 includes a large number of
@@ -198,6 +199,7 @@ we can take the sum of ND masked quantities along the ``1`` axis like so::
      array([ True, False]),
      StdDevUncertainty([1.41421356, 1.73205081]))
 
+<<<<<<< HEAD
 .. _whatsnew-5.3-coordinates-refresh-site-registry:
 
 Refresh cached observatory site registry for |EarthLocation| methods
@@ -214,6 +216,16 @@ however, is updated from time to time to include new locations.  The user
 may refresh their locally cached site registry by passing the new
 ``refresh_cache=True`` option to these two functions.
 
+.. _whatsnew-5.3-modeling-measurement-uncertainties:
+
+Support for collapse operations on arbitrary axes in ``nddata``
+===============================================================
+
+Propagate measurement uncertainties into the best-fit parameter covariances
+via the ``weights`` keyword argument in non-linear fitters. Decreasing
+the ``weights`` will now increase the uncertainties on the best-fit parameters.
+
+
 Full change log
 ===============