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.

Author: yoshuawuyts

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