Improve readme

[jimdale]

Added a lot of material:

• a bit on partitioning
• fairly detailed descriptions of model and ensemble scripts and filesystems
• described running models/ensembles, with run-through of command line options

[lujzi05]

• I added one sentence about the difference between running main.py and run.sh
• I updated the link of the glossary to point to the glossary in the docs repo
• I added the description of drift_self_test flag (please check if this flag applies both to single runs and to ensemble runs)

Otherwise it looks good.