README: Clarify that release mode is needed in order to use human-panic

I just spent about twenty minutes debugging why human-panic wasn't working.

I hadn't enabled release mode and had no idea that release mode affected it.

Author: timotree3

README.md

@@ -102,6 +102,12 @@ fn main() {
 }
 ```
 
+It only displays a human-friendly panic message in release mode:
+
+```sh
+$ cargo run --release
+```
+
 ## Installation
 
 ```sh