Add Rhai book.

doc/src/safety/sandbox.md

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+Sand-Boxing - Block Access to External Data
+==========================================
+
+{{#include ../links.md}}
+
+Rhai is _sand-boxed_ so a script can never read from outside its own environment.
+
+Furthermore, an [`Engine`] created non-`mut` cannot mutate any state outside of itself;
+so it is highly recommended that [`Engine`]'s are created immutable as much as possible.
+
+```rust
+let mut engine = Engine::new();     // create mutable 'Engine'
+
+engine.register_get("add", add);    // configure 'engine'
+
+let engine = engine;                // shadow the variable so that 'engine' is now immutable
+```