F OpenNebula/one#6774: Add docs (#508)

Author: vichansson

assets/images/sunstone-tfaenforce-screen.png

@@ -165,6 +165,18 @@ You can get an additional authentication level by using a two-factor authenticat
 
 ![sunstone_2fa_auth](/images/sunstone_login_2fa.png)
 
+### Enforce globally
+
+Setting the `enforce_2fa` option in the `/etc/one/fireedge/sunstone/sunstone-server.conf` configuration file will enforce tfa authentication across all users.
+
+```yaml
+enforce_2fa: true
+```
+
+Attempting to log in without a TFA authenticator set will present the following screen, prompting the user to add one.
+
+![sunstone_tfaenforce_screen](/images/sunstone-tfaenforce-screen.png)
+
 ### Authenticator App
 
 This method requires a token generated by any of these applications: [Google Authentication](https://play.google.com/store/apps/details?id=com.google.android.apps.authenticator2&hl=en), [Authy](https://authy.com/download/), or [Microsoft Authentication](https://www.microsoft.com/en-us/p/microsoft-authenticator/9nblgggzmcj6?activetab=pivot:overviewtab).