Added description of "bit slip" copy protection, and screenshot of an Easter egg.

* Fleshed out the description of the "bit slip" copy protection which was most probably used in 5.25" POP. (I think I understand the method at a theoretical level, at any rate...) The protection checks are present in `YELLOW.MAIN.S`, and also elsewhere.
* Added commentary on the code unscrambling in `PURPLE.S`.
* Added a screenshot of the "confusion" Easter egg, as it appeared inside a (patched) LinApple emulator session. There are at least 3 other Easter eggs.