Update release-notes.md

Author: Ben Hillis

WSL/release-notes.md

@@ -10,6 +10,11 @@ ms.localizationpriority: high
 
 # Release Notes for Windows Subsystem for Linux
 
+## Build 19640
+For general Windows information on build 19640 visit the [Windows blog](https://blogs.windows.com/windowsexperience/2020/06/03/announcing-windows-10-insider-preview-build-19640/).
+
+* [WSL2] Stability improvements for virtio-9p (drvfs).
+
 ## Build 19555
 For general Windows information on build 19555 visit the [Windows blog](https://blogs.windows.com/windowsexperience/2020/01/30/announcing-windows-10-insider-preview-build-19555/).