docs/recursive: add documentation hints for recursive structures

Closes #220

askama/src/lib.rs

@@ -402,6 +402,29 @@
 //!
 //! Askama supports block comments delimited by `{#` and `#}`.
 //!
+//!
+//! ## Recursive Structures
+//!
+//! Recursive implementations should preferably use a custom iterator and
+//! use a plain loop. If that is not doable, call `.render()`
+//! directly by using an expression as shown below.
+//! Including self does not work, see #105 and #220 .
+//!
+//! ```rust
+//! use askama::Template;
+//!
+//! #[derive(Template)]
+//! #[template(source = r#"
+//! //! {% for item in children %}
+//!    {{ item.render().unwrap() }}
+//! {% endfor %}
+//! "#, ext = "html", escape = "none")]
+//! struct Item<'a> {
+//!     name: &'a str,
+//!     children: &'a [Item<'a>],
+//! }
+//! ```
+//!
 //! # Optional functionality
 //!
 //! ## Rocket integration