Add note about opt-level surprising results

Author: johnthagen

README.md

@@ -65,6 +65,14 @@ which optimizes the binary for **speed**. To instruct Cargo to optimize for mini
 opt-level = "z"  # Optimize for size.
 ```
 
+Note that in some cases the `"s"` level may result in a smaller binary than `"z"`, as explained in
+the
+[`opt-level` documentation](https://doc.rust-lang.org/cargo/reference/profiles.html#opt-level):
+
+> It is recommended to experiment with different levels to find the right balance for your project.
+There may be surprising results, such as ... the `"s"` and `"z"` levels not being necessarily
+smaller. 
+
 # Enable Link Time Optimization (LTO)
 
 ![Minimum Rust: 1.0](https://img.shields.io/badge/Minimum%20Rust%20Version-1.0-brightgreen.svg)