Betterttv: Add documentation for public API endpoints

[RAnders00]

Third-party applications, bots, websites etc. often find the need to integrate with BetterTTV, and in the past they've used the v2 API for that with great success.

It would be great to hear:

• When the old (https://api.betterttv.net/2/) API is being deprecated, and when it will be removed,
• and what endpoints of the new API (https://api.betterttv.net/3/) third-party devs are supposed and allowed to be used

I'm mostly looking at /cached/users/twitch/:channelID and /cached/emotes/global in my applications.

It would be great to have a .md file in this repository, or a page on the website to link to, as a reference for the v3 API, if that was possible, and maybe some guidelines for consuming and implementing against the API.

[night]

Currently we don't provide maintenance/support for usage of our API in other applications, which is why we don't document our APIs. Using any of our APIs is currently at your own risk, and could break/change at any time (though that's not something we try to do, hence versioning).

v2 will be removed soon, but there are still some internal services using v2 (which is the only reason it remains online).

While we still don't officially support v3, you could consider the endpoints used within our extension to be deemed as safer to use (which are the cached ones you noted).