Add a note about minimal required nightly version

blog/content/second-edition/posts/02-minimal-rust-kernel/index.md

@@ -269,6 +269,12 @@ build-std = ["core", "compiler_builtins"]
 
 This tells cargo that it should recompile the `core` and `compiler_builtins` libraries. The latter is required because it is a dependency of `core`. In order to recompile these libraries, cargo needs access to the rust source code, which we can install with `rustup component add rust-src`.
 
+<div class="note">
+
+**Note:** The `unstable.build-std` configuration key requires at least the Rust nightly from 2020-07-15. Right now, the `rustfmt` component is [not available](https://rust-lang.github.io/rustup-components-history/) on these recent nightlies, so you might need to use `rustup update nightly --force` to update your nightly, which skips the `rustfmt` component if it's not available.
+
+</div>
+
 After setting the `unstable.build-std` configuration key and installing the `rust-src` component, we can rerun the our build command:
 
 ```