Updated kernel release notes

Author: craigloewen-msft

WSL/kernel-release-notes.md

@@ -11,6 +11,11 @@ ms.localizationpriority: high
 
 We've added support for WSL 2 distributions, [which use a full Linux kernel](https://devblogs.microsoft.com/commandline/shipping-a-linux-kernel-with-windows/). This Linux kernel is open source, with its source code available at the [WSL2-Linux-Kernel](https://github.com/microsoft/WSL2-Linux-Kernel) repository. This Linux kernel is delivered to your machine via Microsoft Update, and follows a separate release schedule to the Windows Subsystem for Linux which is delivered as part of the Windows image.
 
+## 5.4.91
+*Release Date*: Prerelease 2021/02/22
+
+[Official Github release link](https://github.com/microsoft/WSL2-Linux-Kernel/releases/tag/linux-msft-5.4.91)
+
 ## 5.4.72
 *Release Date*: 2021/01/21