Add Rhai book.

doc/src/language/modules/rust.md

@@ -0,0 +1,30 @@
+Create a Module from Rust
+========================
+
+{{#include ../../links.md}}
+
+To load a custom module (written in Rust) into an [`Engine`], first create a [`Module`] type,
+add variables/functions into it, then finally push it into a custom [`Scope`].
+
+This has the equivalent effect of putting an [`import`] statement at the beginning of any script run.
+
+```rust
+use rhai::{Engine, Scope, Module, i64};
+
+let mut engine = Engine::new();
+let mut scope = Scope::new();
+
+let mut module = Module::new();             // new module
+module.set_var("answer", 41_i64);           // variable 'answer' under module
+module.set_fn_1("inc", |x: i64| Ok(x+1));   // use the 'set_fn_XXX' API to add functions
+
+// Push the module into the custom scope under the name 'question'
+// This is equivalent to 'import "..." as question;'
+scope.push_module("question", module);
+
+// Use module-qualified variables
+engine.eval_expression_with_scope::<i64>(&scope, "question::answer + 1")? == 42;
+
+// Call module-qualified functions
+engine.eval_expression_with_scope::<i64>(&scope, "question::inc(question::answer)")? == 42;
+```