Enable plugins to listen on updates of plugin project config

Change-Id: I6dbb45ed79385ae7299fb83992072dab57485332
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
diff --git a/Documentation/dev-plugins.txt b/Documentation/dev-plugins.txt
index 2b07cf7..c27811a 100644
--- a/Documentation/dev-plugins.txt
+++ b/Documentation/dev-plugins.txt
@@ -710,6 +710,10 @@
 }
 ----
 
+By overwriting the `onUpdate` method of `ProjectConfigEntry` plugins
+can be notified when this configuration parameter is updated on a
+project.
+
 [[project-specific-configuration]]
 == Project Specific Configuration in own config file