Update identity.md

Author: Rick-Anderson

aspnetcore/security/authentication/identity.md

@@ -227,6 +227,7 @@ To prevent publishing static Identity assets (stylesheets and JavaScript files f
 
 ## Next Steps
 
+* [ASP.NET Core Identity source code](https://github.com/dotnet/aspnetcore/tree/master/src/Identity)
 * See [this GitHub issue](https://github.com/dotnet/AspNetCore.Docs/issues/5131) for information on configuring Identity using SQLite.
 * [Configure Identity](xref:security/authentication/identity-configuration)
 * <xref:security/authorization/secure-data>