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