Document how to properly export an icon from Sketch

Due to multiple icons with color issues in the past, we should properly document how to export from Sketch. Where should we do that? In this Repo or design.gitlab.com?

/cc @sarrahvesselov

The following discussion from !174 (merged) should be addressed:

• @leipert started a discussion: (+3 comments)

  @cperessini It has a fixed white background and a fixed black foreground (those fill=) Could you make remove that? Otherwise we cannot change the color of the icon in CE! Please do not forget to run yarn run svg after fixing it 😄

  cc/ @andyvolpe Should we write guidelines for UX how to properly export an icon?