Update events doc for event.which usage on keyboard events

[vernonk]

When using fireEvent.keyDown the event.which value is a fallback of 0 unless the keyCode is passed. I thought this might be helpful for other folks trying the same.

e.g.

fireEvent.keyDown(domNode, { key: 'Enter', code: 13, charCode: 13 }); // event.which => 0

fireEvent.keyDown(domNode, { key: 'Enter', keyCode: 13 }); // event.which => 13

[kentcdodds]

Cool, thanks!

[kentcdodds]

@all-contributors please add @vernonk for docs

[allcontributors]

@kentcdodds

I've put up a pull request to add @vernonk! 🎉

[vernonk]

Awesome. Thanks! :)