Add new authorization details to README

[LeviHassel]

This PR adds more details about Spotify authorization to the README based on the discussion in #37.

I've personally used every approach I mentioned for Flutter, but I'd appreciate any thoughts from @rinukkusu and @roisnir on how to better explain the Authorization Code Flow for a Dart app.

[roisnir]

This PR adds more details about Spotify authorization to the README based on the discussion in #37.

I've personally used every approach I mentioned for Flutter, but I'd appreciate any thoughts from @rinukkusu and @roisnir on how to better explain the Authorization Code Flow for a Dart app.

First, this documentation is great and definitely going to help users thank you!

Unfortunately, I haven't written such dart app too, and I have no good idea for production apps but here are some workarounds ideas.
If the case is testing an api request in an example file of a flutter app (like @hayribakici described here) or any other command-line dart application. You can launch the authorization URL in a web browser (see this thread) or print and paste to the browser. Sign in, and then, when redirected copy the redirect URL with the authorization code and paste to the app's input (with stdin.readLineSync())

In the case of an HTTP server, I can't think of a way to authorize because it requires user interface for the user to authenticate.

[LeviHassel]

@roisnir thanks for the ideas! I think the command-line approach you described would be a much quicker solution for @hayribakici than what I suggested here.

[LeviHassel]

I just updated the PR with code snippets for the url_launcher/uni_links approach and the WebView approach @roisnir mentioned. I also hid all of the imaginary function explanations inside an expandable/collapsible section so that it doesn't detract from all of the other good advice in the README. 😄

[rinukkusu]

Thanks for the great efforts you put into making this documentation better for everyone!