add reason example

Co-authored-by: Eric Huss <[email protected]>

Author: Mark-Simulacrum

posts/2024-09-05-Rust-1.81.0.md

@@ -68,7 +68,13 @@ if code runs in an environment without floating point support, you could use
 Clippy to lint on such usage with `#![deny(clippy::float_arithmetic)]`.
 However, if a new developer to the project sees this lint fire, they need to
 look for (hopefully) a comment on the deny explaining why it was added. With
-Rust 1.81, they can be informed directly in the compiler message:
+Rust 1.81, they can be informed directly in the compiler message by adding a `reason` to the lint attribute:
+
+```rust
+#![deny(clippy::float_arithmetic, reason = "no hardware float support")]
+```
+
+When this lint is triggered, the compiler will include the reason in its output:
 
 ```text
 error: floating-point arithmetic detected