update README

on the following HPC's/Clusters:

• Ithaca (IC3 machine)
• Stargate (IC3 machine)
• MareNostrum (BSC machine)
• MareNostrum3 (BSC machine)
• HECToR (EPCC machine)
• Lindgren (PDC machine)
• C2A (ECMWF machine)
• CCA (ECMWF machine)
• ARCHER (EPCC machine)

--> instead talk about SAGA