Update README.md

Author: Ravi Chande

README.md

@@ -20,6 +20,11 @@ The C# extension is powered by [OmniSharp](https://github.com/OmniSharp/omnishar
 * [Documentation](https://code.visualstudio.com/docs/languages/csharp)
 * [Video Tutorial compiling with .NET Core](https://channel9.msdn.com/Blogs/dotnet/Get-started-VSCode-Csharp-NET-Core-Windows)
 
+### What's new in 1.21.3
+
+* Added support for `CheckForOverflowUnderflow ` in csproj files
+* Improved support for .NET Core 3
+
 ### What's new in 1.21.2
 
 * Added a `omnisharp.enableEditorConfigSupport` setting to enable support for .editorconfig.