add additional issues to changelog

CHANGES.rst

@@ -20,6 +20,13 @@ Unreleased
     :issue:`2638`
 -   Cache the help option generated by the ``help_option_names`` setting to
     respect its eagerness. :pr:`2811`
+-   Replace uses of `os.system` with `subprocess.Popen`. :issue:`1476`
+-   Exceptions generated during a command will use the context's ``color``
+    setting when being displayed. :issue:`2193`
+-   Error message when definig option with invalid name is more descriptive.
+    :issue:`2452`
+-   Refactor code generating default ``--help`` option to deduplicate code.
+    :pr:`2563`
 
 
 Version 8.1.7