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!

Author: bors

README.md

@@ -51,10 +51,10 @@ program, and cannot run all programs:
 
 ## Using Miri
 
-Install Miri via `rustup`:
+Install Miri on Rust nightly via `rustup`:
 
 ```sh
-rustup component add miri
+rustup +nightly component add miri
 ```
 
 If `rustup` says the `miri` component is unavailable, that's because not all