add derived docs too

Author: Rich-Harris

documentation/docs/02-runes/03-$derived.md

@@ -45,3 +45,9 @@ Sometimes you need to create complex derivations that don't fit inside a short e
 ```
 
 In essence, `$derived(expression)` is equivalent to `$derived.by(() => expression)`.
+
+## Understanding dependencies
+
+Anything read synchronously inside the `$derived` expression (or `$derived.by` function body) is considered a _dependency_ of the derived state. When the state changes, the derived will be marked as _dirty_ and recalculated when it is next read.
+
+To exempt a piece of state from being treated as a dependency, use [`untrack`](svelte#untrack).