Add Rhai book.

doc/src/rust/indexers.md

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+Custom Type Indexers
+===================
+
+{{#include ../links.md}}
+
+A custom type can also expose an _indexer_ by registering an indexer function.
+
+A custom type with an indexer function defined can use the bracket '`[]`' notation to get a property value.
+
+Indexers are disabled when the [`no_index`] feature is used.
+
+```rust
+#[derive(Clone)]
+struct TestStruct {
+    fields: Vec<i64>
+}
+
+impl TestStruct {
+    fn get_field(&mut self, index: i64) -> i64 {
+        self.fields[index as usize]
+    }
+    fn set_field(&mut self, index: i64, value: i64) {
+        self.fields[index as usize] = value
+    }
+
+    fn new() -> Self {
+        TestStruct { fields: vec![1, 2, 3, 4, 5] }
+    }
+}
+
+let engine = Engine::new();
+
+engine.register_type::<TestStruct>();
+
+engine.register_fn("new_ts", TestStruct::new);
+
+// Shorthand: engine.register_indexer_get_set(TestStruct::get_field, TestStruct::set_field);
+engine.register_indexer_get(TestStruct::get_field);
+engine.register_indexer_set(TestStruct::set_field);
+
+let result = engine.eval::<i64>("let a = new_ts(); a[2] = 42; a[2]")?;
+
+println!("Answer: {}", result);                     // prints 42
+```
+
+For efficiency reasons, indexers **cannot** be used to overload (i.e. override) built-in indexing operations for
+[arrays] and [object maps].