Update events doc for event.which usage on keyboard events (testing-library#262)

Author: vernonk

docs/dom-testing-library/api-events.md

@@ -70,6 +70,10 @@ fireEvent.keyDown(domNode, { key: 'Enter', code: 13 })
 // will Fire an KeyboardEvent with charCode = 0
 fireEvent.keyDown(domNode, { key: 'Enter', code: 13 })
 
+// If using event.which, be sure to set the keyCode or it will be fallback to 0
+// will Fire a KeyboardEvent with expected which = 13
+fireEvent.keyDown(domNode, { key: 'Enter', keyCode: 13 })
+
 // will Fire an KeyboardEvent with charCode = 65
 fireEvent.keyDown(domNode, { key: 'A', code: 65, charCode: 65 })
 ```