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
1 file changed
tree: 92fdcfa796939da49b4ac779fd1f706f11c100ce
  1. src/
  2. pom.xml
  3. README.txt