added note for installation with composer

README.md

 # Adyen_Magento2
 Adyen Payment plugin for Magento2
+
+
+## Install through composer
+
+Make sure you have "minimum-stability": "dev" in your magento composer.json set.
+Because the repositories are private you need to do the folowing:
+
+1. composer config repositories.adyen-adyen-magento2 vcs git@github.com:Adyen/adyen-magento2.git 
+2. composer config repositories.adyen-php-api-library vcs git@github.com:Adyen/adyen-php-api-library.git
+3. composer require adyen/module-payment
+
+## API Library
+Not yet used but communication through the API will go through this library. If you not have access to this repo please let us know.