Add Rhai book.

doc/src/language/print-debug.md

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+`print` and `debug`
+===================
+
+{{#include ../links.md}}
+
+The `print` and `debug` functions default to printing to `stdout`, with `debug` using standard debug formatting.
+
+```rust
+print("hello");         // prints hello to stdout
+print(1 + 2 + 3);       // prints 6 to stdout
+print("hello" + 42);    // prints hello42 to stdout
+debug("world!");        // prints "world!" to stdout using debug formatting
+```
+
+Override `print` and `debug` with Callback Functions
+--------------------------------------------------
+
+When embedding Rhai into an application, it is usually necessary to trap `print` and `debug` output
+(for logging into a tracking log, for example) with the `Engine::on_print` and `Engine::on_debug` methods:
+
+```rust
+// Any function or closure that takes an '&str' argument can be used to override
+// 'print' and 'debug'
+engine.on_print(|x| println!("hello: {}", x));
+engine.on_debug(|x| println!("DEBUG: {}", x));
+
+// Example: quick-'n-dirty logging
+let logbook = Arc::new(RwLock::new(Vec::<String>::new()));
+
+// Redirect print/debug output to 'log'
+let log = logbook.clone();
+engine.on_print(move |s| log.write().unwrap().push(format!("entry: {}", s)));
+
+let log = logbook.clone();
+engine.on_debug(move |s| log.write().unwrap().push(format!("DEBUG: {}", s)));
+
+// Evaluate script
+engine.eval::<()>(script)?;
+
+// 'logbook' captures all the 'print' and 'debug' output
+for entry in logbook.read().unwrap().iter() {
+    println!("{}", entry);
+}
+```