differentiates standalone npm install

if there are not enough use-cases for standalone install, feel free to remove section:

## Install react-ros as standalone dev
```
$ npm install 
```

README.md

@@ -3,7 +3,13 @@ Connect your website to your robot!
 
 [![NPM](https://img.shields.io/npm/v/react-ros.svg)](https://www.npmjs.com/package/react-ros) [![JavaScript Style Guide](https://img.shields.io/badge/code_style-standard-brightgreen.svg)](https://standardjs.com)
 
-## Install
+## Add react-ros to existing npm project
+
+```
+$ npm install react-ros
+```
+
+## Install react-ros as standalone dev
 
 ```
 $ npm install