NPM usage instructions

Would be really helpful if you added some instruction on how to install and use via npm into the README.

Install:
```
 npm i @procore/procore-iframe-helpers
```

Usage:
```
// esm syntax
import {initialize} from "@procore/procore-iframe-helpers"

// commonjs syntax
var {initialize} = require("@procore/procore-iframe-helpers")
```
