Add line about sliding down scree field

Unfortunately, pull requests that delete or modify lines are not eligible for automatic merging.  If you wish to have this pull request auto-merged, please update it to only add lines.  If you don't think you've modified any lines, remember that I consider changing the whitespace (e.g. deleting a space at the end of a line) a modification, even though it may be hard to tell the difference on GitHub.