Update readme to mention that maybe it's necessary to be logged in now

Hi. I can't get an access token. I checked the code, and from what I understand the flow is:
1) with the consumer key, request a request_token
2) user auths with the given request_token on the browser
3) after user authenticates, request an access_token

I tried reproducing this flow manually (calling all the involved functions by hand) and it's clear that the problem arises when requesting the access_token, the response is always a 403 (forbidden).

Is it possible that the authentication flow of Pocket changed and the authentication of the request_token isn't complete? I checked the authorized apps on my pocket profile and the app is listed there, so it's weird to me that we're getting this 403