Document this artifactory/jenkins plugin...

https://stackoverflow.com/questions/54217470/jenkins-artifactory-plugin-with-conanclient-huge-buildinfo-refering-unrelated-p

When they reuse in Jenkins workspace, the `$CONAN_USER_HOME/conan_log.log` is kept, so the build info is filled with past information from previous jobs. Add docs commenting that the file, and in general, the conan home, should be wiped, if the user wants to keep the data, the file should be removed.