Update README to include note for oauth token.

"Note" is now a required parameter for creating an authorization.
https://developer.github.com/v3/oauth_authorizations/#create-a-new-authorization

The current example throws an error because ```note``` is missing.

```
Octokit::UnprocessableEntity: POST https://api.github.com/authorizations: 422 - Validation Failed
Error summary:
  resource: OauthAccess
  code: missing_field
  field: description // See: https://developer.github.com/v3/oauth_authorizations/#create-a-new-authorization
```