Document on UI how to add tags for articles

UI should say "press Enter key" to add tags, as it is not very intuitive.

Search terms: How to add tags for articles? Can't add tags for articles. Answer: use the Enter key

And note that it is broken on current live React demo BTW: https://github.com/gothinkster/react-redux-realworld-example-app/issues/151#issuecomment-808417846 for new post creation, but already fixed on master.