Properly document how to export an icon from Sketch

Andy Volperequested to merge
36-document-how-to-properly-export-an-icon-from-sketch into master

This improves the documentation on how to add an icon/illustration, by:

  • Adding how to export an icon from Sketch
  • Consolidating the fragmented instructions we currently have in the README.md and CONTRIBUTING.md

Closes #36 (closed)

Edited by Lukas Eipert

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