clarify why we need to use the derived rune

Author: stephane-vanraes

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

@@ -2,7 +2,19 @@
 title: $derived
 ---
 
-Derived state is declared with the `$derived` rune:
+
+Svelte's `<script>` blocks are run only when the component is created, so assignments within a `<script>` block are not automatically run again when a prop updates. 
+
+```svelte
+<script>
+	let count = $state(0);  
+	// this will only set `doubled` on component creation
+	// it will not update when `count` does
+	let doubled = count * 2;
+</script>
+```
+
+We can make any variable based on others reactive by declaring them a Derived state using the `$derived` rune:
 
 ```svelte
 <script>