Update README with module functions API.

README.md

@@ -2012,6 +2012,8 @@ with `export` statements that _exports_ certain global variables and functions a
 
 Everything exported as part of a module is constant and read-only.
 
+### Importing modules
+
 A module can be _imported_ via the `import` statement, and its members accessed via '`::`' similar to C++.
 
 ```rust
@@ -2039,6 +2041,33 @@ crypto::encrypt(others);    // <- this causes a run-time error because the 'cryp
                             //    is no longer available!
 ```
 
+### Creating custom modules from Rust
+
+To load a custom module 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;
+```
+
 Script optimization
 ===================