Warn that you can't use repl.it on Day 4

Pygame can't be used in repl.it, so I've added a warning and linked to
the installation instructions.
Basic outline for the lesson
Add some sample code to the program
Add finished Day4 sample code
Add a small explanation of the coordinate grid
Make the coordinate grid image an actual image
Flesh out the descriptions a bit
Merge pull request #3 from carter-sande/day4

Day 4 - PyGame random art materials