added a few lines to specify how to set user/pass for javagal GUI

Added 2 notes to README.

We may also want to review [GalGuiHttpApplication.java](https://github.com/ismb/jemma/blob/master/jemma.osgi.javagal.gui/src/main/java/org/energy_home/jemma/javagal/gui/GalGuiHttpApplication.java) to check how users are authorized. We correctly use the OSGi UserAdmin service, but the current implementation is not really 100% readable ...