Add config option to exclude fields from auth providers sync

doc/en_US/config.markdown

@@ -1,4 +1,4 @@
-Config file
+Config File
 ===========
 
 You can customize the default settings of Kanboard by adding a file `config.php` at the project root or in the `data` folder.
@@ -273,7 +273,7 @@ define('LOG_FILE', __DIR__.DIRECTORY_SEPARATOR.'data'.DIRECTORY_SEPARATOR.'debug
 ```
 
 Brute-force protection
----------------------
+----------------------
 
 ```php
 // Enable captcha after 3 authentication failure
@@ -338,4 +338,7 @@ define('TOTP_ISSUER', 'Kanboard');
 
 // Maximum number of events stored in the table "project_activities"
 define('PROJECT_ACTIVITIES_MAX_EVENTS', 10000);
+
+// Comma separated list of fields to not synchronize when using external authentication providers
+define('EXTERNAL_AUTH_EXCLUDE_FIELDS', 'username');
 ```