Add writeup on plugins.

doc/src/rust/modules/create.md

@@ -0,0 +1,55 @@
+Create a Module from Rust
+========================
+
+{{#include ../../links.md}}
+
+
+Create via Plugin
+-----------------
+
+By far the simplest way to create a [module] is via a [plugin module].
+
+
+Create via `Module` API
+-----------------------
+
+Manually creating a [module] is possible via the `Module` API.
+
+For the complete `Module` API, refer to the [documentation](https://docs.rs/rhai/{{version}}/rhai/struct.Module.html) online.
+
+
+Make the `Module` Available to the `Engine`
+------------------------------------------
+
+In order to _use_ a custom module, there must be a [module resolver].
+
+The easiest way is to use, for example, the [`StaticModuleResolver`][module resolver] to hold such
+a custom module.
+
+```rust
+use rhai::{Engine, Scope, Module, i64};
+use rhai::module_resolvers::StaticModuleResolver;
+
+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
+
+// Create the module resolver
+let mut resolver = StaticModuleResolver::new();
+
+// Add the module into the module resolver under the name 'question'
+// They module can then be accessed via: 'import "question" as q;'
+resolver.insert("question", module);
+
+// Set the module resolver into the 'Engine'
+engine.set_module_resolver(Some(resolver));
+
+// Use module-qualified variables
+engine.eval::<i64>(&scope, r#"import "question" as q; q::answer + 1"#)? == 42;
+
+// Call module-qualified functions
+engine.eval::<i64>(&scope, r#"import "question" as q; q::inc(q::answer)"#)? == 42;
+```