Make Cro approach and "purpose" clearer

[Altai-man]

See https://colabti.org/irclogger/irclogger_log/perl6?date=2018-12-07#l879

With all respect to current design, I think, there should be a clearer note or explanation that:

• Cro is not just your average web framework just because it can serve some http.
• While showing "end user" features is nice compared to "abstract benefit" ones("You can build a great things with it, "just" that"), there are tons of web frameworks, so Cro::Core key value is being unnoticed by possible end users as in "Oh, they deal with Web, but I don't do that, I do $task-where-composable-chains-are-great-to-use, I don't need a web server".

We already have Approach document written, but one should search for it.