Update readme for plugin-paths.

Author: markstory

README.md

@@ -1,13 +1,11 @@
-CakePHP Plugin Installer
-========================
+# CakePHP Plugin Installer
 
 A composer installer for installing CakePHP 3.0+ plugins.
 
 This installer ensures your application is aware of CakePHP plugins installed
 by composer in `vendor/`.
 
-Usage
------
+## Usage
 
 Your CakePHP application should already depend on `cakephp/plugin-installer`, if
 not in your CakePHP application run:
@@ -23,6 +21,21 @@ only need to specify the `type` in their composer config:
 "type": "cakephp-plugin"
 ```
 
+## Multiple Plugin Paths
+
+If your application uses multiple plugin paths. In addition to configuring your
+application settings you will also need to update your `composer.json` to ensure
+the generated `cakephp-plugins.php` file is correct:
+
+```
+// Define the list of plugin-paths your application uses.
+"extra": {
+    "plugin-paths": ["plugins", "extra_plugins"]
+}
+```
+
+## Plugin Setup
+
 For the installer to work properly ensure that your plugin's composer config
 file has a proper autoload section. Assuming your plugin's namespace is "MyPlugin"
 the autoload section would be like: