docs(README): add note about @types/google.maps (#565)

closes #564

Author: jpoehnelt

README.md

@@ -19,12 +19,6 @@ Available via npm as the package [@googlemaps/js-api-loader](https://www.npmjs.c
 npm i @googlemaps/js-api-loader
 ```
 
-or
-
-```sh
-yarn add @googlemaps/js-api-loader
-```
-
 Alternatively you may add the umd package directly to the html document using the unpkg link.
 
 ```html
@@ -33,6 +27,14 @@ Alternatively you may add the umd package directly to the html document using th
 
 When adding via unpkg, the loader can be accessed at `google.maps.plugins.loader.Loader`.
 
+### TypeScript 
+
+TypeScript users need to install the following types package.
+
+```sh
+npm i -D @types/google.maps
+```
+
 ## Documentation
 
 The reference documentation can be found at this [link](https://googlemaps.github.io/js-api-loader/index.html). The Google Maps JavaScript API [documentation](https://developers.google.com/maps/documentation/javascript/tutorial) is the authoritative source for the loader options.