add docs for nested vuex modules

Author: chrisvfritz

docs/state.md

@@ -1,7 +1,9 @@
 # State management
 
-- [Modules](#modules)
-- [Helpers](#helpers)
+- [State management](#state-management)
+  - [Modules](#modules)
+  - [Helpers](#helpers)
+  - [Module Nesting](#module-nesting)
 
 ## Modules
 
@@ -26,3 +28,39 @@ export default {
   },
 }
 ```
+
+## Module Nesting
+
+Vuex modules can be nested, which sometimes makes sense for organizational purposes. For example, if you created these files:
+
+```js
+// @file src/state/modules/dashboard.js
+
+export const state = {
+  role: 'project-manager',
+}
+```
+
+```js
+// @file src/state/modules/dashboard/videos.js
+
+export const state = {
+  all: [],
+}
+
+export const getters = {
+  favorited(state) {
+    return state.all.filter((video) => video.favorited)
+  },
+}
+```
+
+Then you'd be able to access those modules with:
+
+```js
+store.state.dashboard.role
+store.state.dashboard.videos.all
+store.getters['dashboard/videos/favorited']
+```
+
+As you can see, placing the `videos` module in a folder called `dashboard` automatically nests it underneath the `dashboard` namespace. This works even if a `dashboard.js` file doesn't exist. You can also have as many levels of nesting as you want.