Add configuration info to Facebook Connect plugin readme.txt as suggested by Nicolás Galdo - refs CT#8156

plugin/add_facebook_login_button/readme.txt

@@ -3,4 +3,9 @@ README
 This plugin adds a button to allow users to log into Chamilo with their Facebook account.<br/><br/>
 To display this button on your portal, you have to enable the Facebook authentification and configure it.<br/>
 To enable and configure the Facebook authentification on your Chamilo platform, go to Administration > Configuration settings > Facebook<br/>
+You should then add the App ID and the Secret Key provided by Facebook inside the app/config/auth.conf.php file<br />
+Finally, you will have to set the following line in your app/config/configuration.php<br />
+<pre>
+$_configuration['facebook_auth'] = 1;
+</pre>
 This plugin has been developed to be added to the login_top or login_bottom region in Chamilo, but you can put it in whichever region you want.<br/>