Added css import instructions to readme

[seanrankin]

I noticed you're getting some questions about importing CSS in the issues, so I added the instructions to the Readme.

Great little component. Thanks man!

[codecov-io]

Codecov Report

Merging #33 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #33   +/-   ##
=======================================
  Coverage   80.35%   80.35%           
=======================================
  Files           2        2           
  Lines         112      112           
=======================================
  Hits           90       90           
  Misses         22       22

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