docs(website): add updated derived docs

Author: christianalfoni

packages/overmind-website/api/derive.md

@@ -1,7 +1,14 @@
 # Derive
 
+You can add derived state to your application. You access derived state like any other value, there is not need to call it as a function. The derived value is cached and will only update when any accessed state changes.
+
 ```marksy
 h(Example, { name: "api/derive" })
 ```
 
-You can add derived state to your application. You access derived state like any other value, there is not need to call it as a function. The derived value is cached and will only update when any accessed state changes. 
+The function defining your derived state receives two arguments. The first argument is the object the derived function is attached to. Ideally you use this argument to produce your derived state, though you can access the second argument which is the root state of the application. The root state allows you to access any state.
+
+
+```marksy
+h(Notice, null, "Accessing **rootState** might cause unnecessary updates to the derived function as it will track more state, though typically not an issue")
+```

packages/overmind-website/examples/api/derive.ts

@@ -13,7 +13,7 @@ export type Item = {
 
 export const items: Item[] = []
 
-export const completedItems: Derive<Item[]> = state =>
+export const completedItems: Derive<Item[]> = (state, rootState) =>
   state.items.filter(item => item.completed)
     `,
         },
@@ -26,7 +26,7 @@ import * as derived from './derived'
 
 export const items = []
 
-export const completedItems = state =>
+export const completedItems = (state, rootState) =>
   state.items.filter(item => item.completed)
     `,
         },