README: -concurrentUnstableThreshold, +optional un/pw for auth

check_jenkins_job_extended.pl had concurrentUnstableThreshold
removed in f503c7a1beafb071b44f7954a29c5818a92ffc0d, but it's
still documented. Also added optional username & password.
Merge pull request #7 from davestern/README-remove-unstable-arg

Updated README to reflect current arguments