Update README with ES6 import example.

Add ES6 import syntax for `jsonld` in README.

The other import styles will not work with Node.js, so this marks out
the happy path for Node devs.

Author: BigBlueHat

README.md

@@ -94,6 +94,10 @@ npm install jsonld
 const jsonld = require('jsonld');
 ```
 
+```js
+import jsonld from 'jsonld';
+```
+
 ### Browser (bundler) + npm
 
 ```