README: document -p option.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
README: update section on standalone.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
README: add note about JRuby.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>