Add note about rustfmt fix

Author: Mark-Simulacrum

posts/2021-07-29-Rust-1.54.0.md

@@ -67,6 +67,10 @@ TODO
 There are other changes in the Rust 1.54.0 release:
 check out what changed in [Rust](XXX), [Cargo](XXX), and [Clippy](XXX).
 
+rustfmt has also been fixed in the 1.54.0 release to properly format nested
+out-of-line modules. This may cause changes in formatting to files that were
+being ignored by the 1.53.0 rustfmt. See details [here](https://github.com/rust-lang/rust/pull/86424).
+
 ### Contributors to 1.54.0
 
 Many people came together to create Rust 1.54.0.