replace link to common repo with page describing common classes

Author: gregor-j

common.md

@@ -0,0 +1,32 @@
+## Common classes and exceptions
+
+The following common classes are available in namespace `phpsap\common`, and the exceptions are available in namespace `phpsap\exceptions`. See the [php-sap/common repository on github][common].
+
+### Config
+
+* `AbstractConfigContainer` A JSON encodeable configuration container implementing PSR-11 for retrieving configuration parameters.
+* `AbstractConfig` Configure basic connection parameters for SAP remote function calls, that are common to both connection types (A, and B)  implementing `\phpsap\interfaces\IConfig`.
+* `AbstractConfigA` Configure connection parameters for SAP remote function calls using a specific SAP application server (type A) implementing `\phpsap\interfaces\IConfigA`.
+* `AbstractConfigB` Configure connection parameters for SAP remote function calls using load balancing (type B) implementing `\phpsap\interfaces\IConfigB`.
+
+### Connection
+
+* `AbstractConnection` Manages a PHP/SAP connection instance implementing `\phpsap\interfaces\IConnection`.
+* `ConnectionPool` Provides a static pool of available PHP/SAP connections. The connections are organized using their connection IDs. The connection ID ensures, that only one connection per configuration is established.
+
+### Function
+
+* `AbstractFunction` Manages a PHP/SAP remote function call implementing `\phpsap\interfaces\IFunction`.
+* `AbstractRemoteFunctionCall` Proxy class to simplify PHP/SAP remote function calls implementing `\phpsap\interfaces\IFunction`.
+
+### Exceptions
+
+* `SapException` a generic exception inherited by all other exception.
+* `ConfigKeyNotFoundException` in case a configuration key could not be found.
+* `IncompleteConfigException` in case `generateConfig()` cannot get all mandatory keys for the configuration.
+* `ConnectionFailedException` in case a connection could not be established.
+* `UnknownFunctionException` in case the remote function doesn't exist.
+* `FunctionCallException` in case the remote function call failed.
+
+
+[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."

index.md

@@ -13,11 +13,8 @@ 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.
 
-The [common classes and exceptions][common] add logic that is not specific to
+The [common classes and exceptions](common) add logic that is not specific to
  the underlying PHP module.
 
 The module specific implementations contain code that is specific to [the
  underlying PHP-module](php-modules).
-
-
-[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."