Readme: Update Atom instructions with support for css...

[rstacruz]

The Atom snippet only works for <styled jsx>{...} snippets. This adds support for css`...` as well.

Here's how I have it configured (pardon the font ligatures):

I also have it configured for source.css.postcss since I often use styled-jsx-postcss, but I didn't note that in this PR.

[rstacruz]

Here's how it looks like:

Which would otherwise look like this:

[giuseppeg]

awesome thank you!