Sync CSS example snippet in README

[ybiquitous]

Which issue, if any, is this issue related to?

Ref: #266

Is there anything in the PR that needs further explanation?

I've just copied and pasted the content of __tests__/valid.css:

stylelint-config-standard/__tests__/valid.css

Line 1 in 5c0e054

@import url("x.css");