Update: Credits; Allow exit with 'q'

Key 'q' can now be pressed during the credits sequence to end the game.
Note if not fullscreen, corner x can also be clicked to quit.
Update: Rules now rendered text

Rules now set in /res/text/rules.txt and rendered instead of being taken
from an image.

rules.png deleted and its reference removed from resmaps.
Docs update: To version 0.81

Also updated /res/text/instructions.py to include instructions for
writing rules.