Provide a `::date-time` spec equivalent to clj-time's

[jivagoalves]

First of all, thanks for the library!

We (@estikado and I) are in the process of migrating an application from clj-time to clojure.java-time. Given we use Clojure spec, we were wondering if you folks would be interested in having an equivalent of clj-time's spec.

We can open a PR in the next days if that's ok for you.

Thanks!

[dm3]

Sure, that would we be great!

Although these days I'd encourage people to evaluate cljc.java-time first before using my library - it has the JS story covered. I'll add the endorsement to the README.