add Usage.md

docs/Usage.md

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+Usage
+=====
+
+# Basic usage
+
+Use the `Es\ControllerPlugins\PluginsTrait` to obtain the functionality of 
+plugins in your controller:
+```
+use Es\ControllerPlugins\PluginsTrait;
+
+class ExampleController
+{
+    use PluginsTrait;
+
+    public function fooAction()
+    {
+        $result = $this->forward('Foo.Bar.Baz', 'index');
+        // ...
+    }
+}
+```
+
+# The custom plugins
+
+The module can provide configuration of custom controller plugins. By convention,
+this configuration should be located in a separate file `controller-plugins.config.php`
+in the configuration directory of module. This file must be included with the
+system configuration file:
+```
+project/
+    module/
+        ExampleModule/
+            Module.php
+            config/
+                system.config.php
+                controller-plugins.config.php
+            src/
+            ...
+```
+
+The file `project/module/ExampleModule/config/system.config.php`:
+```
+return [
+    'controller_plugins' => require __DIR__ . DIRECTORY_SEPARATOR . 'controller-plugins.config.php',
+];
+```
+
+The file `project/module/ExampleModule/config/controller-plugins.config.php`:
+```
+return [
+    'foo' => 'ExampleModule\ControllerPlugin\FooPlugin',
+    'bar' => 'ExampleModule\ControllerPlugin\BarPlugin',
+    // ...
+];
+``` 
+
+Then you can use a custom plugin in the usual way:
+```
+use Es\ControllerPlugins\PluginsTrait;
+
+class ExampleController
+{
+    use PluginsTrait;
+
+    public function indexAction()
+    {
+        $foo = $this->foo();
+        $bar = $this->bar();
+        // ...
+    }
+}
+```
+If the plugin is callable, `$this->foo()` from example above will returns the 
+result of call the plugin. Otherwise, it will returns the instance of plugin.