update changelog for 5.1.0 release and remove news fragment files (#277)

* update changelog for 5.1.0 release and remove news fragment files

* Add release description in changelog

Author: aaronayres35

CHANGES.txt

@@ -1,6 +1,24 @@
 Apptools CHANGELOG
 ==================
 
+Version 5.1.0
+~~~~~~~~~~~~~
+
+Released : 2021-01-13
+
+This is a minor release in which the modules in the apptools.undo subpackage are
+modified to import from pyface.undo rather than redefining the classes.  This
+should help ease the transition to using pyface.undo in place of the now
+deprecated apptool.undo.
+
+Deprecations
+------------
+* Import from pyface.undo.* instead of redefining classes in apptools.undo.* (#272)
+
+Documentation changes
+---------------------
+* Add module docstrings to the various api modules in apptools subpackages (#274)
+
 Version 5.0.0
 ~~~~~~~~~~~~~