Add writeup on plugins.

doc/src/rust/packages/plugin.md

@@ -0,0 +1,41 @@
+Load a Plugin Module as a Package
+================================
+
+{{#include ../../links.md}}
+
+[Plugin modules] can be loaded as a package just like a normal [module].
+
+```rust
+use rhai::Engine;
+use rhai::plugin::*;
+
+// Define plugin module.
+#[export_module]
+mod my_module {
+    pub fn greet(name: &str) -> String {
+        format!("hello, {}!", name)
+    }
+    pub fn get_num() -> i64 {
+        42
+    }
+}
+
+fn main() {
+    let mut engine = Engine::new();
+
+    // Create plugin module.
+    let module = exported_module!(my_module);
+
+    // Make the 'greet' and 'get_num' functions available to scripts.
+    engine.load_package(module);
+}
+```
+
+
+Share a Package Among `Engine`s
+------------------------------
+
+Loading a [module] via `Engine::load_package` consumes the module and it cannot be used again.
+
+If the functions are needed for multiple instances of [`Engine`], create a [custom package] from the
+[plugin module], which can then be shared with `Package::get`.