Clarify CSS url conversions in README

[mrj]

Propshaft's README currently has the following text to describe how URLs in CSS files are converted:

Propshaft will automatically convert asset references in CSS to use the digested file names. So background: url("/bg/pattern.svg") is converted to background: url("/assets/bg/pattern-2169cbef.svg") before the stylesheet is served.

This only works if "bg" is a subdirectory of an asset path like "images". If "bg" is it's own top-level asset directory, url("pattern.svg") (with or without a leading slash) must instead be written. Similarly, the principle of least surprise path url("images/pattern.svg") won't usually work for assets in "images". So, most-commonly, such CSS URLs won't contain slashes, making the "bg" example misleading.

Suggested text: The path given in a CSS URL must be the path of that asset relative to one of the registered asset directory paths. [examples...]