Add concise explanation of the difference between String, Runes and Characters

[filiph]

This might belong to https://github.com/flutter/website, but (imho) not before package:characters lands with a stable API (1.0.0+).

Maybe something like:

	List<int>	String	Runes	Characters
Also known as	bytes	code units	code points	grapheme clusters
Level	lowest (data)	low (default Dart)	???	highest
Usage	sending	keys, computer-readable names, ...	???	human text
Example	stdin .listen(...)	json["id"] == "deadbeef"	???	input.characters .replaceAll(...)

But of course, much better than this.

Related:

• Dart Strings should support Unicode grapheme cluster operations language#34
• Support grapheme cluster aware operations via a package language#685

[mit-mit]

1.0 is in progress in dart-archive/characters#18 :-)

But I still like the idea of providing this overview here in this package's README. WDYT @lrhn ?

[lrhn]

Lets do something: dart-archive/characters#19