message: Update readme with min requirements
diff: # xero-node
  ![npm](https://img.shields.io/npm/v/xero-node?label=xero-node)
  +## Requirements
  +node version 10.13.0 or higher
  +
  ## Release of SDK with oAuth 2 support
  Version 4.x of Xero NodeJS SDK only supports oAuth2 authentication and the following API sets.
  * accounting
  