Document use of 0.1 as the "time step" in run-life in Game of Life

[NicMcPhee]

If you use the default "time step" for big-bang (1/28th of a second), it just can't keep up and the display is fairly jerky. You also can't really visually track what's happening. I changed that to 0.1 in my code, but I never said anything about that in the write-up for part 4 of the Game of Life project, and I should.