Add doc for board configuration

docs/board-configuration.markdown

@@ -0,0 +1,30 @@
+Board configuration
+===================
+
+Some parameters for the boards can be changed with a config file.
+
+Default values are available in the file `config.default.php`.
+If you want to override the default values, you have to create a config file `config.php` in the root directory of your Kanboard installation.
+
+### Auto-refresh frequency for the public board view
+
+```php
+// Auto-refresh frequency in seconds for the public board view (60 seconds by default)
+define('BOARD_PUBLIC_CHECK_INTERVAL', 60);
+```
+
+### Auto-refresh frequency for the board (Ajax polling)
+
+```php
+// Board refresh frequency in seconds (the value 0 disable this feature, 10 seconds by default)
+define('BOARD_CHECK_INTERVAL', 10);
+```
+
+### Task highlighting
+
+Display a shadow around the task when a task was moved recently. Set the value 0 to disable this feature.
+
+```php
+// Period (in seconds) to consider a task was modified recently (0 to disable, 2 days by default)
+define('RECENT_TASK_PERIOD', 48*60*60);
+```