add interface descriptions to readme

Author: gregor-j

README.md

@@ -1,10 +1,20 @@
 # PHP/SAP interfaces
 
-[![License: MIT][license-mit]](LICENSE)
-
 **ATTENTION: THIS IS WORK IN PROGRESS AND NOT TO BE USED UNTIL THIS MESSAGE DISAPPEARS!**
 
+[![License: MIT][license-mit]](LICENSE)
+
 This repository defines interfaces for implementing the [PHP/SAP][phpsap] API.
 
+## Interfaces
+
+The following interfaces are available in namespace `phpsap\interfaces`:
+
+* `IConfig` Configure basic connection parameters for SAP remote function calls, that are common to both connection types (A, and B).
+* `IConfigA` Configure connection parameters for SAP remote function calls using a specific SAP application server (type A).
+* `IConfigB` Configure connection parameters for SAP remote function calls using load balancing (type B).
+* `IConnection` Manage a PHP/SAP connection instance.
+* `IFunction` Manage a PHP/SAP remote function call.
+
 [phpsap]: https://php-sap.github.io
 [license-mit]: https://img.shields.io/badge/license-MIT-blue.svg