Add reference to get access token on the my apps page

[duncan]

As suggested by Eric Mutta in https://disqus.com/home/discussion/wunderlistdeveloper/wunderlist_developer_38/#comment-2164292599

You should probably update these docs to indicate that one can get the Access Token on the MY APPS page of Wunderlist.com - this is SOOOO MUCH EASIER than doing the OAuth dance: a user just logs in, generates the access token then pastes it into the relevant app (in my case a client-side desktop app with no server-side component to supply the redirect URL).