Replace screenshot with wider screenshot of art

Problem: I didn't really like random text posts from me being in the
readme, and the aspect ratio of the screenshot meant that it ended up
taking up lots of vertical space.

Solution: Use a wide screenshot of some concept art to keep it from
stretching vertically very far while still showing off the absolute
basics of Oasis.

README.md

@@ -5,7 +5,7 @@ This is an experimental client built with HTML, CSS, and Node.js without any
 front-end JavaScript. The goal is to support basic social messaging schemas with
 some extra tools for debugging, _not_ to support all known message types.
 
-![screenshot of oasis](./screenshot.png)
+![Screenshot of Oasis](./docs/screenshot.png)
 
 ## Usage