readme: how to use with react-native 📝

Hopefully prevents situations like in #56 in the future.

#56 (comment)

readme.md

@@ -30,6 +30,10 @@ There's [a company called HaCon](http://hacon.de) that sells [a public transport
 npm install hafas-client
 ```
 
+### with [react-native](https://facebook.github.io/react-native/)
+
+`hafas-client` as well its dependencies use [Node-builtin modules](https://nodejs.org/dist/latest/docs/api/) and [Node globals](https://nodejs.org/api/globals.html). To be able to use it within react-native, follow [the instructions at `node-libs-react-native`](https://github.com/parshap/node-libs-react-native/blob/master/README.md#usage).
+
 
 ## API