Auto merge of #2147 - RalfJung:readme, r=RalfJung
Auto merge of #2147 - RalfJung:readme, r=RalfJung

split flag section into common and advanced flags

As discussed with `@oli-obk` . However I was not always sure which flags to put where, so if you think some flags should be in the other category please let me know. :)

rust-lang/miri,rust-lang/miri,rust-lang/miri,rust-lang/miri