Update README regarding the 'artifact.directed' parameter

Since we now have issue artifact networks, the 'artifact.directed'
parameter now is effective for issue artifact networks.

Signed-off-by: Thomas Bock <bockthom@cs.uni-saarland.de>

README.md

@@ -632,7 +632,7 @@ Updates to the parameters can be done by calling `NetworkConf$update.variables(.
     * possible values: [*`"cochange"`*, `"callgraph"`, `"mail"`, `"issue"`]
 - `artifact.directed`
     * The directedness of edges in an artifact network
-    * **Note**: This parameter does not take effect for now, as the `cochange` relation is always undirected, while the `callgraph` relation is always directed. For the other relations (`mail` and `issue`), we currently do not have data available to exhibit edge information.
+    * **Note**: This parameter does only affect the `issue` relation, as the `cochange` relation is always undirected, while the `callgraph` relation is always directed. For the `mail`, we currently do not have data available to exhibit edge information.
   * [`TRUE`, *`FALSE`*]
 - `edge.attributes`
     * The list of edge-attribute names and information