Add Rhai book.

doc/src/safety/max-modules.md

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+Maximum Number of Modules
+========================
+
+{{#include ../links.md}}
+
+Rhai by default does not limit how many [modules] can be loaded via [`import`] statements.
+
+This can be changed via the `Engine::set_max_modules` method. Notice that setting the maximum number
+of modules to zero does _not_ indicate unlimited modules, but disallows loading any module altogether.
+
+A script attempting to load more than the maximum number of modules will terminate with an error result.
+
+This check can be disabled via the [`unchecked`] feature for higher performance
+(but higher risks as well).
+
+```rust
+let mut engine = Engine::new();
+
+engine.set_max_modules(5);      // allow loading only up to 5 modules
+
+engine.set_max_modules(0);      // disallow loading any module (maximum = zero)
+
+engine.set_max_modules(1000);   // set to a large number for effectively unlimited modules
+```