message: updates readme tokenSet expiration example
diff: @@ -315,19 +315,19 @@ await xero.buildConsentUrl()
  // tokenSet and its expiration
  const tokenSet = await xero.readTokenSet();
  -const now = new Date().getTime()
  -if (tokenSet.expires_in > now) {
  +tokenSet.expired() // returns boolean true/false
  +tokenSet.expires_in // returns seconds
  +tokenSet.expires_at // returns milliseconds
  +new Date(tokenSet.expires_at * 1000).toLocaleString()) // readable expiration
  +
  +if (tokenSet.expired()) {
  const validTokenSet = await xero.refreshToken()
  // or you can refresh the token without needing to initialize the openid-client
  // helpful for background processes where you want to limit any dependencies
  await xero.refreshWithRefreshToken(client_id, client_secret, tokenSet.refresh_token)
  }
  -tokenSet.expires_in // returns seconds
  -tokenSet.expires_at // returns milliseconds
  -new Date(tokenSet.expires_at * 1000).toLocaleString()) // readable expiration
  -
  // some endpoints date fields require
  // the MS date format for POST'ing data
  const dateString = "1990-02-05"
  