Update README.md to reflect correct usage of "import"

[nickcmaynard]

There are a number of issues: #281, #292, #242 which appear to be falling down on the import not working correctly.

The imports in these issues use:
import QRCode from 'qrcode'

But this results in QRCode as being undefined.

As #281 and #186 state, this import appears to work:
import * as QRCode from 'qrcode'

Ideally, the documented method would work, but it doesn't. Thus this PR updates the documentation to show the method that does work.

[mitra42]

At the same time as doing this, it would be good to document a way to import in the browser. so that it can be used inside modules. For example
import * as QRCode from '../qrcode/build/qrcode.js'

imports fine but QRCode is a module, so for example there is no QRCode.toCanvas

[geeky01adarsh]

Thank you for pointing it out, it worked for me.

[mitra42]

For use in the browser, I moved to "qrcode-generator-es6" as I couldn't get this one to work.

My examples using that module (which failed in using this one because of the problem above) are in https://github.com/mitra42/html-element-extended in qrelementextended.js - just the display part - the detect part still has some browser compability issues I'm working on.

Note - ... not a criticism of this module, I posted my response before realizing the name of this module (node-qrcode) suggests its to run in the server under node, not in the browser.