Add notes about 'sling' dir not deleted by mvn clean

git-svn-id: https://svn.apache.org/repos/asf/incubator/sling/trunk@741173 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/README.txt b/README.txt
index bbe213b..5ed48bf 100644
--- a/README.txt
+++ b/README.txt
@@ -41,6 +41,8 @@
 
 How to run this
 ---------------
+  NOTE: "mvn clean" does not delete the "sling" work directory - make sure to 
+  delete it manually if you want to start from a clean state.
 
 1) Build all Sling bundles