Added 4.6.4 release notes.

Author: Boy Baukema

docs/release_notes/4.6.4.md

@@ -0,0 +1,9 @@
+# OpenConext EngineBlock v4.6.4 Release Notes #
+
+Push Metadata improvements.
+
+* Downgrade notice about multivalued attributes to info
+* [Remove `libxml_disable_entity_loader()` call #180](https://github.com/OpenConext/OpenConext-engineblock/issues/180)
+* [error while pushing metadata #212](https://github.com/OpenConext/OpenConext-engineblock/issues/212)
+* [Metadata Push: DisplayName missing #223](https://github.com/OpenConext/OpenConext-engineblock/issues/223)
+* Added metadata push test client on `/api/connections/test`.