Auto merge of #2547 - RalfJung:fake-apis, r=saethlin
Auto merge of #2547 - RalfJung:fake-apis, r=saethlin

note on deterministic 'fake' API implementations

Now that we also have fake clocks, it seems prudent to warn about this at the top of the readme.

r? `@rust-lang/miri`

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