Document what Atom IS

[madorb]

There was this thread a couple of years ago: #91

but it doesn't look like any action was taken. As someone who just spent 10 minutes attempting to suss out what exactly Atom is via google (and was totally foiled due to the atom editor) before searching through the issues here, I think it would be very nice to actually include what atom is somewhere on the site in text. Happy to write a blurb based on what i gleaned from that that thread and put it in a PR - just don't know where you'd want it to go (main readme? readme in atom subfolder?)