Update README.md to clarify url(...) references in CSS in processed CSS

[jasonfb]

@jrochkind

adds language to README: "It no longer necessary to use eg asset-url() helper (which was introduced by sass-rails). All url(...) references in CSS are just transparently processed to use appropriate digested filenames automatically now."

[jasonfb]

https://github.com/rails/sprockets-rails/pull/476

[januszm]

It'd be also great to include this information in https://github.com/rails/dartsass-rails (especially the part about changing image-url to just url)