Auto merge of #1148 - yoshuawuyts:patch-1, r=RalfJung
Auto merge of #1148 - yoshuawuyts:patch-1, r=RalfJung

Document miri can only be installed on nightly

This updates the installation instructions to point out that miri is only available on nightly, and encodes this assumption into the installation instructions.

It took me a bit of research to find out *why* miri wasn't installing on `windows-msvc`:
```txt
C:\Users\yoshu> rustup component add miri
error: component 'miri' for target 'x86_64-pc-windows-msvc' is unavailable for download for channel stable
```

I thought it must 've been something with the `msvc` target, but it turned out I was running on `stable`, and miri can only be installed on `nightly`. Having that be clearer in the installation instructions would've saved me some time debugging. Thanks!

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