Add jemalloc section admonition

Author: johnthagen

README.md

@@ -94,10 +94,11 @@ lto = true
 ![Minimum Rust: 1.28](https://img.shields.io/badge/Minimum%20Rust%20Version-1.28-brightgreen.svg)
 ![Maximum Rust: 1.31](https://img.shields.io/badge/Maximum%20Rust%20Version-1.31-brightgreen.svg)
 
-As of Rust 1.32,
-[`jemalloc` is removed by default](https://blog.rust-lang.org/2019/01/17/Rust-1.32.0.html).
-**If using Rust 1.32 or newer, no action is needed to reduce binary size regarding this
-feature**.
+> [!IMPORTANT]
+> As of Rust 1.32,
+> [`jemalloc` is removed by default](https://blog.rust-lang.org/2019/01/17/Rust-1.32.0.html).
+> **If using Rust 1.32 or newer, no action is needed to reduce binary size regarding this
+> feature**.
 
 **Prior to Rust 1.32**, to improve performance on some platforms Rust bundled
 [jemalloc](https://github.com/jemalloc/jemalloc), an allocator that often