replace link to interfaces repo with page describing interfaces

Author: gregor-j

index.md

@@ -9,7 +9,7 @@ PHP/SAP defines a layer of abstraction between actual SAP remote function calls
 
 [![PHP/SAP](res/php-sap.svg)](res/php-sap.svg)
 
-The [interfaces][interfaces] define a common denominator on how to configure a
+The [interfaces](interfaces) define a common denominator on how to configure a
  connection to SAP, prepare a SAP remote function call, and invoke a SAP remote
  function call.
 
@@ -20,5 +20,4 @@ The module specific implementations contain code that is specific to [the
  underlying PHP-module](php-modules).
 
 
-[interfaces]: https://github.com/php-sap/interfaces "Interfaces for implementing the PHP/SAP API."
 [common]: https://github.com/php-sap/common "Exceptions and abstract classes containing logic for PHP/SAP that is not specific to the underlying PHP module."

interfaces.md

@@ -0,0 +1,29 @@
+## Interfaces
+
+The following PHP/SAP interfaces are available in namespace `phpsap\interfaces`. See the [php-sap/interfaces repository on github][interfaces].
+
+### Config
+
+The configuration interfaces are separated into a generic SAP configuration, that contains connection parameters that are common to both connection types (A and B), and connection type specific SAP configuration.
+
+* `IConfig` Configure basic connection parameters for SAP remote function calls, that are common to both connection types (A, and B) and require a `generateConfig()` method, that returns the specific configuration structure of the underlying PHP module.
+* `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).
+
+### Connection
+
+* `IConnection` Manage a PHP/SAP connection instance.
+
+### Function
+
+* `IFunction` Manage a PHP/SAP remote function call.
+
+### Exceptions
+
+* `ISapException` Generic SAP exception.
+* `IIncompleteConfigException` The configuration is incomplete.
+* `IConnectionFailedException` The SAP connection failed.
+* `IUnknownFunctionException` The requested remote function could not be found.
+* `IFunctionCallException` The SAP remote function call failed.
+
+[interfaces]: https://github.com/php-sap/interfaces "Interfaces for implementing the PHP/SAP API."