Consolidate list of access token management

[stevenschobert]

Opening up this issue to consolidate and improve the documentation around managing access tokens.

This is by far the biggest barrier to getting started with Instafeed.js, and is unfortunately much not complicated in v2.

In v1, there used to be an link in the readme that would immediately create an access token and show it to the developer so they could get started.

In v2, a server side component is required, so I don't know if we can get down to a single click -- but we do have options ranging from "doing it yourself", to using something pre-built by the community like instant-tokens.

[chrishappy]

Should we just post different providers?

https://www.instant-tokens.com - £1.49per month