Add info on details tag

[jtemporal]

I saw this tweet:

😲I just learned that GitHub allows you create drop-downs. Here's the result of `<details> (very long stacktrace) </details>`: pic.twitter.com/lkM2ABi2Je

— Felix Rieseberg (@felixrieseberg) April 4, 2017

from @felixrieseberg and I think this cheat sheet needs this info, including the gif =D

I'm opening this issue because I want to add it, but only will have time to get around to it later ;)