Add documentation for metadata

ChangeLog

@@ -10,14 +10,14 @@ Breaking changes:
 New features:
 
 * Add CSV import for users and tasks
+* Add Task, User and Project metadata for plugin creators
 
 Improvements:
 
 * Allow to change comments sorting
 * Add the possibility to append or not custom filters
 * Make mail transports pluggable
-* Add Task, User and Project metadata for plugin creators
+* Do not show scroll-bars when a column is collapsed on Windows systems
-* Do not show scroll-bars when a column is collapsed (IE)
 
 Bug fixes:
 

doc/plugin-metadata.markdown

@@ -0,0 +1,38 @@
+Metadata
+========
+
+You can attach metadata for each project, task and user.
+Metadata are custom fields, it's a key/value table.
+
+By example your plugin can store external information for a task or new settings for a project.
+Basically that allow you to exend the default fields without having to create new tables.
+
+Attach metadata to tasks
+------------------------
+
+```php
+
+// Return a dictionary of metadata (keys/values) for the $task_id
+$this->taskMetadata->getAll($task_id);
+
+// Get a value only for a task
+$this->taskMetadata->get($task_id, 'my_plugin_variable', 'default_value');
+
+// Return true if the metadata my_plugin_variable exists
+$this->taskMetadata->exists($task_id, 'my_plugin_variable');
+
+// Create or update metadata for the task
+$this->taskMetadata->save($task_id, ['my_plugin_variable' => 'something']);
+```
+
+Metadata types
+--------------
+
+- TaskMetadata: `$this->taskMetadata`
+- ProjectMetadata: `$this->projectMetadata`
+- UserMetadata: `$this->userMetadata`
+
+Notes
+-----
+
+- Always prefix the metadata name with your plugin name

doc/plugins.markdown

@@ -13,6 +13,7 @@ Plugin creators should specify explicitly the compatible versions of Kanboard. I
 - [Add schema migrations for plugins](plugin-schema-migrations.markdown)
 - [Add mail transports](plugin-mail-transports.markdown)
 - [Add notification types](plugin-notifications.markdown)
+- [Attach metadata to users, tasks and projects](plugin-metadata.markdown)
 
 Examples of plugins
 -------------------