Suggest installing types as dev dependencies (#163)

Typescript types are usually only required at build time, and not runtime.
But the old instructions would end up with the types being around at run-time.

From the [npm install documentation](https://docs.npmjs.com/cli/v6/commands/npm-install)

> `npm install` saves any specified packages into `dependencies` by default.

README.md

@@ -54,7 +54,7 @@ npm install @koa/router
 ## Typescript Support
 
 ```sh
-npm install @types/koa__router
+npm install --save-dev @types/koa__router
 ```