modify readme

Author: gregor-j

README.md

@@ -1,2 +1,10 @@
-# interfaces
-PHP/SAP interfaces
+# PHP/SAP interfaces
+
+[![License: MIT][license-mit]](LICENSE)
+
+**ATTENTION: THIS IS WORK IN PROGRESS AND NOT TO BE USED UNTIL THIS MESSAGE DISAPPEARS!**
+
+This repository defines interfaces for implementing the [PHP/SAP][phpsap] API.
+
+[phpsap]: https://php-sap.github.io
+[license-mit]: https://img.shields.io/badge/license-MIT-blue.svg