docs(README): fix link to CSS Usage page

[jahvi]

The CSS Usage link markdown wasn't being processed, I just added an empty line before to make this work.

[welcome]

💖 Thanks for opening this pull request! 💖

Here is a list of things that will help get it across the finish line:

• Run yarn lint locally to catch formatting errors. This will fix some errors automatically, commit and push any changes.
• Run yarn test locally to catch errors. This ensures all components still behave as they should.
• Run yarn start to run the doc site locally and try a few pages, ensuring everything is in good working order.
• Include tests when adding/changing behavior.

We get a lot of pull requests on this repo, so please be patient and we will get back to you as soon as we can.

[codecov-io]

Codecov Report

Merging #3212 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3212   +/-   ##
=======================================
  Coverage   99.92%   99.92%           
=======================================
  Files         169      169           
  Lines        2790     2790           
=======================================
  Hits         2788     2788           
  Misses          2        2

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[welcome]

Congrats on merging your first pull request! 🎉🎉🎉

[layershifter]

❤️

[levithomason]

Released in semantic-ui-react@0.83.0.