Update docs regarding modules.

doc/src/rust/modules/imp-resolver.md

@@ -0,0 +1,56 @@
+Implement a Custom Module Resolver
+=================================
+
+{{#include ../../links.md}}
+
+For many applications in which Rhai is embedded, it is necessary to customize the way that modules
+are resolved.  For instance, modules may need to be loaded from script texts stored in a database,
+not in the file system.
+
+A module resolver must implement the trait [`rhai::ModuleResolver`][traits],
+which contains only one function: `resolve`.
+
+When Rhai prepares to load a module, `ModuleResolver::resolve` is called with the name
+of the _module path_ (i.e. the path specified in the [`import`] statement). Upon success, it should
+return a [`Module`]; if the module cannot be load, return `EvalAltResult::ErrorModuleNotFound`.
+
+Example
+-------
+
+```rust
+use rhai::{ModuleResolver, Module, Engine, EvalAltResult};
+
+// Define a custom module resolver.
+struct MyModuleResolver {}
+
+// Implement the 'ModuleResolver' trait.
+impl ModuleResolver for MyModuleResolver {
+    // Only required function.
+    fn resolve(
+        &self,
+        engine: &Engine,    // reference to the current 'Engine'
+        path: &str,         // the module path
+        pos: Position,      // location of the 'import' statement
+    ) -> Result<Module, Box<EvalAltResult>> {
+        // Check module path.
+        if is_valid_module_path(path) {
+            // Load the custom module.
+            let module: Module = load_secret_module(path);
+            Ok(module)
+        } else {
+            Err(Box::new(EvalAltResult::ErrorModuleNotFound(path.into(), pos)))
+        }
+    }
+}
+
+let mut engine = Engine::new();
+
+// Set the custom module resolver into the 'Engine'.
+engine.set_module_resolver(Some(MyModuleResolver {}));
+
+engine.consume(r#"
+    import "hello" as foo;  // this 'import' statement will call
+                            // 'MyModuleResolver::resolve' with "hello" as path
+    foo:bar();
+"#)?;
+```