Additional documentation
Additional documentation

This change adds some addition clarifying documentation about cleaning before
package builds.

[resolves #410]

svenfrauen/java-buildpack,rakutentech/java-buildpack,hmlingesh/java-buildpack,violetagg/tomee-buildpack,violetagg/java-buildpack,pivotal-cf/weblogic-buildpack,cloudfoundry/java-buildpack,lucasreginato/custom-java-buildpack,cloudfoundry-community/tomee-buildpack,Appdynamics/java-buildpack,dsyer/java-buildpack,svenfrauen/java-buildpack,demo-app/java-buildpack,cloudfoundry-community/jboss-buildpack,tom-collings/jboss-buildpack,rakutentech/java-buildpack,dsyer/java-buildpack,ariel-bentu/java-buildpack,waltersve/java-buildpack,stipx/java-buildpack,martinSaad/cfJavaBuildPack_newRelic,cloudfoundry-community/jboss-buildpack,afalak/java-buildpack,pivotal-cf/weblogic-buildpack,hmlingesh/java-buildpack,Appdynamics/java-buildpack,PatrikSteuer/java-buildpack,violetagg/tomee-buildpack,hmlingesh/java-buildpack,stipx/java-buildpack,tom-collings/jboss-buildpack,demo-app/java-buildpack,hmlingesh/java-buildpack,cloudfoundry/java-buildpack,martinSaad/cfJavaBuildPack_newRelic,afalak/java-buildpack,ariel-bentu/java-buildpack,pivotal-cf/weblogic-buildpack,pivotal-cf/weblogic-buildpack,waltersve/java-buildpack,cloudfoundry-community/tomee-buildpack,lucasreginato/custom-java-buildpack,violetagg/java-buildpack,PatrikSteuer/java-buildpack