Document Blazor UI: Authentication

docs/en/UI/Blazor/Authentication.md

@@ -1,3 +1,11 @@
 # Blazor UI: Authentication
 
-TODO
+The [application startup template](../../Startup-Templates/Application.md) is properly configured to use OpenId Connect to authenticate the user through the server side login form;
+
+* When the Blazor application needs to authenticate, it is redirected to the server side.
+* Users can enter username & password to login if they already have an account. If not, they can use the register form to create a new user. They can also use forgot password and other features. The server side uses IdentityServer4 to handle the authentication.
+* Finally, they are redirected back to the Blazor application to complete the login process.
+
+This is a typical and recommended approach to implement authentication in Single-Page Applications. The client side configuration is done in the startup template, so you can change it.
+
+See the [Blazor Security document](https://docs.microsoft.com/en-us/aspnet/core/blazor/security) to understand and customize the authentication process.