Document that -s followed by -f does not work as expected unless using -Zmtime-on-use

[Ploppz]

https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#mtime-on-use

What I mean is, in the README you should mention that this flags needs to be set. It takes a while to debug why it doesn't work, until finally someone on Discard could point me to that flag.

[Ploppz]

Btw: I didn't try that yet. I'm on linux. And the behaviour I've seen without that flag at least, is:

• let's say build is cached. Then the next cargo build doesn't touch the files used
• then the next -f will remove everything
• so next cargo build rebuilds everythin
• so that the next -f doesn't remove anything we need
• so that the next cargo build has everything cached.

And so it alternates.

[wagenet]

@holmgr ping