Merge pull request #671 from timotree3/patch-1

Add alternative for how to install if a change in nightly broke the rustup component

Author: RalfJung

README.md

@@ -53,6 +53,8 @@ Install Miri via `rustup`:
 rustup component add miri
 ```
 
+If `rustup` says the `miri` component is unavailable, that's because not all nightly releases come with all tools. Check out [this website](https://rust-lang.github.io/rustup-components-history) to determine a nightly version that comes with Miri and install that, e.g. using `rustup install nightly-2019-03-28`.
+
 Now you can run your project in Miri:
 
 1. Run `cargo clean` to eliminate any cached dependencies.  Miri needs your