More detailed README and small bugfix

[ckritzinger]

This adds quite a lot of detail to the README, I think it's basically everything that I would have needed to get Hello World up and running.

I'm not sure how you feel about about having a longer readme versus moving more detail to examples/. Personally I find it quite useful to have the main gist of things in a Readme that I can read from top to bottom.

I also fixed a small bug in getExpiryDate that I ran into. JWTs don't use padding but it seems that dart:convert requires padding even for base64Url encoding.

[rkistner]

The issue is that JWTs use base64-url encoding instead of standard base64. Apart from the padding, there are also other minor encoding differences. Could you check if this works?

Suggested change

	// dart:convert doesn't like missing padding
	if (part.length % 4 > 0) {
	part += '=' * (4 - part.length % 4);
	}
	final rawData = base64Decode(part);
	final rawData = base64Url.decode(part);

[ckritzinger]

I'll test, but their docs suggest otherwise: https://api.flutter.dev/flutter/dart-convert/base64Url-constant.html

requires padding

[ckritzinger]

I tested with your suggested change and still get the same FormatException about invalid length.

dart-lang/sdk#39510 seems to suggest this is deliberate.

[ckritzinger]

However, there does seem to be a better way to do this, brb

[ckritzinger]

using https://api.flutter.dev/flutter/dart-convert/Base64Codec/normalize.html seems like a better approach

[rkistner]

Thanks! Turns out my initial suggested change doesn't even make a difference - base64url.decode and base64.decode does the same thing.