docs: add more info in threads

info.toml: 

```toml
[[exercises]]
name = "threads3"
path = "exercises/threads/threads3.rs"
mode = "test"
hint = """
An alternate way to handle concurrency between threads is to use
a mpsc (multiple producer, single consumer) channel to communicate.
With both a sending end and a receiving end, it's possible to
send values in one thread and receive them in another.
Multiple producers are possible by using clone() to create a duplicate
of the original sending end.
See https://doc.rust-lang.org/book/ch16-02-message-passing.html for more info.
"""
```

threads3'hint contains this link, so it should be placed in Further Information

Author: Selflocking

exercises/20_threads/README.md

@@ -7,3 +7,4 @@ Within your program, you can also have independent parts that run simultaneously
 
 - [Dining Philosophers example](https://doc.rust-lang.org/1.4.0/book/dining-philosophers.html)
 - [Using Threads to Run Code Simultaneously](https://doc.rust-lang.org/book/ch16-01-threads.html)
+- [Using Message Passing to Transfer Data Between Threads](https://doc.rust-lang.org/book/ch16-02-message-passing.html)