docs: add typescript instructions (nuxt-community#264)

Author: NickBolles

docs/setup.md

@@ -26,3 +26,24 @@ module.exports = {
   }
 }
 ```
+
+
+### Typescript setup
+
+Add the types to your "types" array in tsconfig.json after the `@nuxt/vue-app` entry
+
+**tsconfig.json**
+
+```json
+{
+  "compilerOptions": {
+    "types": [
+      "@nuxt/vue-app",
+      "@nuxtjs/axios"
+    ]
+  }
+}
+```
+> **Why?**
+>
+> Because of the way nuxt works the `$axios` property on the context has to be merged into the nuxt `Context` interface via [declaration merging](https://www.typescriptlang.org/docs/handbook/declaration-merging.html). Adding `@nuxtjs/axios` to your types will import the types from the package and make typescript aware of the additions to the `Context` interface.