reverse-string: point out limited crates (#1958)

exercises/practice/reverse-string/.docs/instructions.append.md

@@ -2,16 +2,19 @@
 
 ## Bonus
 
-Test your function on this string: `uüu` and see what happens. Try to write a function that properly
-reverses this string. Hint: grapheme clusters
+Test your function on this string: `uüu` and see what happens.
+Try to write a function that properly reverses this string.
+Hint: grapheme clusters
 
-To get the bonus test to run, remove the ignore flag (`#[ignore]`) from the
-last test, and execute the tests with:
+To get the bonus test to run, remove the ignore flag (`#[ignore]`) from the last test, and execute the tests with:
 
 ```bash
-$ cargo test --features grapheme
+cargo test --features grapheme
 ```
 
-You will need to use external libraries (a `crate` in rust lingo) for the bonus task. A good place to look for those is [crates.io](https://crates.io/), the official repository of crates.
+You will need to use external libraries (a `crate` in rust lingo) for the bonus task.
+A good place to look for those is [crates.io](https://crates.io/), the official repository of crates.
+Please remember that only a limited set of crates is supported by our test runner.
+The full list is in [this file](https://github.com/exercism/rust-test-runner/blob/main/local-registry/Cargo.toml) under the section `[dependencies]`.
 
 [Check the documentation](https://doc.rust-lang.org/cargo/guide/dependencies.html) for instructions on how to use external crates in your projects.