expand on json encoding/decoding re:dart

[shrugs]

json encoding and decoding is famously complicated in flutter because runtime reflection is disabled, so it'd be good to give readers an indication of their different options—similar to how this page works—in order to avoid a situation where someone expects that json is plug-and-play, but then realizes that things are more complicated than they expected.

[lesnitsky]

@shrugs thanks, this is completely valid point, will add more details to json example