API: OAuth 2.0, new authentication mechanism

Since the inception of the Speechify API, we have been using API keys for authentication. While this has worked well for many use-cases, we have realized that it is not the most secure way to authenticate the end-users, especially for the public clients, such as browser-based web apps, as well as native mobile and desktop apps. To harden our security and provide the industry-standard auth mechanism for these scenarios, have implemented OAuth 2.0 as an alternative authentication mechanism. Please read the Authentication Guide for more information on how to use OAuth 2.0 with the Speechify API.