message: adds revocation method to readme helper functions
diff: @@ -341,4 +341,7 @@ await xero.readTokenSet()
  const tokenSet = await xero.readTokenSet()
  await xero.setTokenSet(tokenSet)
  +
  +// You can revoke a user's refresh token and remove all their connections to your app by making a request to the revocation endpoint.
  +await xero.revokeToken()
  ```
  