Add new "runtime values" chapter in the book

book/src/SUMMARY.md

@@ -3,6 +3,7 @@
 - [Introduction](./introduction.md)
 - [Getting started](./getting_started.md)
 - [Creating templates](./creating_templates.md)
+- [Runtime values](./runtime.md)
 - [Debugging](./debugging.md)
 - [Configuration](./configuration.md)
 - [Template syntax](./template_syntax.md)

book/src/runtime.md

@@ -0,0 +1,35 @@
+# Runtime values
+
+It is possible to define variables at runtime and to use them in the templates using the `VALUES`
+variable. To do so you need to use the `_with_values` variants of the `render` methods. It expects
+an extra argument of type `Values`:
+
+```rust
+use rinja::Values;
+
+let mut values = Values::new();
+// We add a new value named "name" with the value "Bibop".
+values.add("name", "Bibop");
+// We add a new value named "another" with a vec.
+values.add("another", vec![false]);
+```
+
+The `Values` type is storing data with the `Any` trait, allowing to store any type as long as it
+implements this trait.
+
+Then to render with these values:
+
+```rust
+TemplateStruct.render_with_values(&values).unwrap();
+```
+
+And to use them in a template:
+
+```jinja
+{% if let Ok(name) = VALUES.get::<&str>() %}
+  name is {{ name }}
+{% endif %}
+```
+
+If you try to retrieve a value with the wrong type or that you didn't set, you will get an
+`Err(ValueError)`.