Update documentation

doc/plugin-registration.markdown

@@ -159,3 +159,23 @@ $this->cli->add(new MyCommand());
 ```
 
 Read the library documentation for more information.
+
+Add new task filters
+--------------------
+
+Since the task lexer is a factory that returns a new instance each time, 
+you have to extend the `taskLexer` container with the method `extend()` of Pimple.
+
+Here is a example:
+
+```php
+public function initialize()
+{
+    $this->container->extend('taskLexer', function($taskLexer, $c) {
+        $taskLexer->withFilter(TaskBoardDateFilter::getInstance($c)->setDateParser($c['dateParser']));
+        return $taskLexer;
+    });
+}
+```
+
+For the filter class implementation, there are several examples in the source code under the namespace `Kanboard\Filter`.

doc/update.markdown

@@ -9,7 +9,7 @@ Important things to do before updating
 --------------------------------------
 
 - **Always make a backup of your data before upgrading**
-- Check that your backup is valid
+- Check that your backup is valid!
 - Always read the [change log](https://github.com/kanboard/kanboard/blob/master/ChangeLog) to check for breaking changes
 - Always close all user sessions (flush all sessions on the server)