Added usage example

Author: vssr

README.md

@@ -1,4 +1,25 @@
 [![Scrutinizer Code Quality](https://scrutinizer-ci.com/g/mediact/composer-file-installer/badges/quality-score.png?b=master)](https://scrutinizer-ci.com/g/mediact/composer-file-installer/?branch=master)
 
 # composer-file-installer
-Install files in a project as part of a `composer install` or `composer update`.
+Install files in a project as part of a `composer install` or `composer update`. Uses the[mediact/file-mapping](https://github.com/mediact/file-mapping) package for moving files according to a source -> destination mapping. The Composer `IOInterface` supplies the file installer with the capabiliy to write the files and supply end-users with output messages.
+
+# Usage example
+```php
+<?php
+// Create a file mapping.
+ $mappingFilePaths = new UnixFileMapping(
+      __DIR__ . '/../folder/files',
+      getcwd(),
+      ['./dir/one','./dir/two']
+      
+  );
+
+// Get a file mapping reader.
+$reader = new UnixFileMappingReader($sourceDirectory, $targetDirectory, $mappingFilePaths);
+
+// Get an installer, supply with the file mapping reader.
+$installer = new FileInstaller($reader);
+
+ // Install according to mapping, supply with Composer IOInterface.
+ $installer->install($io);
+```