Move English documentation to folder en_US

doc/en_US/plugin-group-provider.markdown

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+Custom Group Providers
+======================
+
+Kanboard is able to load groups from an external system.
+This feature is mainly used for project permissions.
+
+Project managers can allow access to a project for a group.
+The end-user will use an auto-complete box and search for a group.
+
+Each time a group query is executed, all registered group providers are executed.
+
+Group Provider Workflow
+-----------------------
+
+1. The end-user start to type the group name in the auto-complete field
+2. The `GroupManager` class will execute the query across all registered group providers
+3. Results are merged and returned to the user interface
+4. After selecting a group, the information of the group are synced to the local database if necessary
+
+Group Provider Interface
+------------------------
+
+Interface to implement: `Kanboard\Core\Group\GroupProviderInterface`.
+
+Classes that implements this interface abstract the group information, there are only 3 methods:
+
+- `getInternalId()`: Get internal database id, return 0 otherwise
+- `getExternalId()`: Get external unique id
+- `getName()`: Get group name
+
+Kanboard will use the external id to sync with the local database.
+
+Group Backend Provider Interface
+--------------------------------
+
+Interface to implement: `Kanboard\Core\Group\GroupBackendProviderInterface`.
+
+This interface requires only one method: `find($input)`.
+The argument `$input` is the text entered from the user interface.
+
+This method must return a list of `GroupProviderInterface`, this is the result of the search.
+
+Backend Registration from Plugins
+---------------------------------
+
+In the method `initialize()` of your plugin register your custom backend like that:
+
+```php
+$groupManager->register(new MyCustomLdapBackendGroupProvider($this->container));
+```
+
+Examples
+--------
+
+- [Group providers included in Kanboard (LDAP and Database)](https://github.com/kanboard/kanboard/tree/master/app/Group)