Incomplete 'chrono' extension documentation

[ghassanpl]

It says the library supports strftime-like date and time formatting but it only seems to support it for old-style time types, and does not mention that. For std::chrono::duration the documentation does not state the valid syntax (and e.g. 'Y' is perhaps non-obviously not supported). There is also no information on the localtime and gmtime functions, even though localtime is used in the example.

[vitaut]

{fmt} supports strftime formatting of both time and durations (https://godbolt.org/z/K8s4Mc) so that part of documentation is correct. Otherwise PRs to improve the docs are welcome.

[ngc92]

At least for std::chrono::duration the strftimeformatting is not really appropriate, because e.g.

fmt::print("strftime-like format: {:%S}\n", 3h + 15min + 30s);

outputs 30 and not 11730. That might be worth pointing out in the documentation.

[vitaut]

Expanded the docs a bit and added the Chrono Format Specifications section. Further improvements are welcome.