Add instructions for fixing the infinitely falling sand issue

[muzikbike]

This bug is dreadful and patching it out makes the Far Lands feasible to explore without requiring killall to be used every single minute. I've also added more detailed instructions for patching out the jitter issue.

Note: these fixes were tested using Beta 1.7.3 via RetroMCP, without Single Player Commands installed. I doubt this changes anything, but you might want to verify these fixes work there just in case.

Fixes #12

[ThisTestUser]

Thanks for the clarifications! I'm actually going to remove the references to the line numbers, as they are different depending on the formatting of the code.