docs: add README

Author: jankal

README.md

@@ -0,0 +1,9 @@
+# Open Source Shopware Plugin Installer
+
+This plugin allows to install all open-source plugins present in the https://packagist.org/ composer registry.
+
+For metadata, https://github.com/nuonic-digital/plugin-data is facilitated.
+
+To install this plugin, download it's latest release from GitHub.
+
+This plugin will also keep itself up to date as it is available through https://packagist.org/ as well.