Move English documentation to folder en_US

doc/en_US/plugin-metadata.markdown

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+Metadata
+========
+
+You can attach metadata for each project, task, user or for the whole application.
+Metadata are custom fields, it's a key/value table.
+
+For example your plugin can store external information for a task or new settings for a project.
+Basically that allow you to extend the default fields without having to create new tables.
+
+Attach metadata to tasks and remove them
+------------------------
+
+```php
+
+// Return a dictionary of metadata (keys/values) for the $task_id
+$this->taskMetadataModel->getAll($task_id);
+
+// Get a value only for a task
+$this->taskMetadataModel->get($task_id, 'my_plugin_variable', 'default_value');
+
+// Return true if the metadata my_plugin_variable exists
+$this->taskMetadataModel->exists($task_id, 'my_plugin_variable');
+
+// Create or update metadata for the task
+$this->taskMetadataModel->save($task_id, ['my_plugin_variable' => 'something']);
+
+// Remove a metadata from a project
+$this->projectMetadataModel->remove($project_id, my_plugin_variable);
+```
+
+Metadata types
+--------------
+
+- TaskMetadata: `$this->taskMetadataModel`
+- ProjectMetadata: `$this->projectMetadataModel`
+- UserMetadata: `$this->userMetadataModel`
+- Settings/Config: `$this->configModel`
+
+Notes
+-----
+
+- Always prefix the metadata name with your plugin name