TASK: Add changelog for 4.0.21

See https://jenkins.neos.io/job/neos-release/228/

Documentation/Appendixes/ChangeLogs/4021.rst

@@ -0,0 +1,41 @@
+`4.0.21 (2019-11-06) <https://github.com/neos/neos-development-collection/releases/tag/4.0.21>`_
+================================================================================================
+
+Overview of merged pull requests
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+`TASK: Update showInvisible check to be upwards compatible <https://github.com/neos/neos-development-collection/pull/2766>`_
+----------------------------------------------------------------------------------------------------------------------------
+
+Uses `$this->request` instead of the http request, to stay upwards compatible with PSR-7 changes.
+
+Related: #2711 
+
+* Packages: ``BaseMixins`` ``Neos``
+
+`TASK : Remove composer requirement for the BaseMixins package itself <https://github.com/neos/neos-development-collection/pull/2760>`_
+---------------------------------------------------------------------------------------------------------------------------------------
+
+The BaseMixin package lists itself as a dependency. This obviously doesn't make any sense (unless there is a very specific reason for this that I'm not aware of).
+
+* Packages: ``BaseMixins`` ``Neos``
+
+`TASK: Allow preview of invisible nodes in live <https://github.com/neos/neos-development-collection/pull/2711>`_
+-----------------------------------------------------------------------------------------------------------------
+
+When opening the preview for a hidden page using the React UI a
+validation error shows up instead of handling the error or showing
+the preview.
+
+This changes fixes that by
+
+- https://github.com/neos/neos-ui/pull/2557
+- and changing Neos to allow invisible nodes to be shown for
+  requests that are granted `Neos.Neos:Backend.GeneralAccess`
+
+See https://github.com/neos/neos-ui/issues/2500
+
+* Packages: ``Browser`` ``Neos``
+
+`Detailed log <https://github.com/neos/neos-development-collection/compare/4.0.20...4.0.21>`_
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~