Add Rhai book.

doc/src/engine/hello-world.md

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+Hello World in Rhai
+===================
+
+{{#include ../links.md}}
+
+To get going with Rhai is as simple as creating an instance of the scripting engine `rhai::Engine` via
+`Engine::new`, then calling the `eval` method:
+
+```rust
+use rhai::{Engine, EvalAltResult};
+
+fn main() -> Result<(), Box<EvalAltResult>>
+{
+    let engine = Engine::new();
+
+    let result = engine.eval::<i64>("40 + 2")?;
+    //                      ^^^^^^^ cast the result to an 'i64', this is required
+
+    println!("Answer: {}", result);             // prints 42
+
+    Ok(())
+}
+```
+
+`rhai::EvalAltResult` is a Rust `enum` containing all errors encountered during the parsing or evaluation process.
+
+
+Evaluate a Script
+----------------
+
+The type parameter is used to specify the type of the return value, which _must_ match the actual type or an error is returned.
+Rhai is very strict here.
+
+Use [`Dynamic`] for uncertain return types.
+
+There are two ways to specify the return type - _turbofish_ notation, or type inference.
+
+```rust
+let result = engine.eval::<i64>("40 + 2")?;     // return type is i64, specified using 'turbofish' notation
+
+let result: i64 = engine.eval("40 + 2")?;       // return type is inferred to be i64
+
+result.is::<i64>() == true;
+
+let result: Dynamic = engine.eval("boo()")?;    // use 'Dynamic' if you're not sure what type it'll be!
+
+let result = engine.eval::<String>("40 + 2")?;  // returns an error because the actual return type is i64, not String
+```
+
+Evaluate a script file directly:
+
+```rust
+let result = engine.eval_file::<i64>("hello_world.rhai".into())?;   // 'eval_file' takes a 'PathBuf'
+```