Update Readme "extendedGrant" function signature

The `extendedGrant` function signature in the Readme documentation does not reflect the implementation in `grant.js`:
```
function useExtendedGrant (done) {
  var self = this;
  this.model.extendedGrant(this.grantType, this.req,
      function (err, supported, user) {
```

I updated the docs to include the first parameter, `grantType`.

Cheers,
Steffen