message: Update README with Invoice as PDF sample code
diff: @@ -202,6 +202,18 @@ xeroClient.core.contacts.getContacts({
  ```
  +
  +Get and Save an Invoice as a PDF:
  +```javascript
  +var invoiceId = "924XXXXXXXXXXX56f";
  + xeroClient.core.invoices.getInvoice(invoiceId, null, null, null,null,"application/pdf")
  + .then(function(data) {
  + fs.writeFile(data.FileName, data.PdfContentRaw, 'binary', function(){
  + // Do something with the file
  + });
  + })
  +```
  +
  ## Docs
  Check the [`docs`](./docs) folder for more detailed examples of how to use each SDK function.
  