Add assignments documentation
//! while `{{ user.name }}` will get the ``name`` field of the ``user``
//! field from the template context.
//!
+//! ## Assignments
+//!
+//! Inside code blocks, you can also declare variables or assign values
+//! to variables.
+//! Assignments can't be imported by other templates.
+//!
+//! Assignments use the let tag:
+//!
+//! ```text
+//! {% let name = user.name %}
+//! {% let len = name.len() %}
+//!
+//! {% let val -%}
+//! {% if len == 0 -%}
+//! {% let val = "foo" -%}
+//! {% else -%}
+//! {% let val = name -%}
+//! {% endif -%}
+//! {{ val }}
+//! ```
+//!
//! ## Filters
//!
//! Values such as those obtained from variables can be post-processed
