Add back security block

Author: victorhuangwq

SECURITY.md

@@ -1,5 +1,11 @@
 <!-- BEGIN MICROSOFT SECURITY.MD V0.0.9 BLOCK -->
 
+## Security
+
+Microsoft takes the security of our software products and services seriously, which includes all source code repositories managed through our GitHub organizations, which include [Microsoft](https://github.com/Microsoft), [Azure](https://github.com/Azure), [DotNet](https://github.com/dotnet), [AspNet](https://github.com/aspnet) and [Xamarin](https://github.com/xamarin).
+
+If you believe you have found a security vulnerability in any Microsoft-owned repository that meets [Microsoft's definition of a security vulnerability](https://aka.ms/security.md/definition), please report it to us as described below.
+
 ## Reporting Security Issues
 
 **Please do not report security vulnerabilities through public GitHub issues.**