docs(Icon): add FontAwesome attribution

[layershifter]

Fixes #1901.

[codecov-io]

Codecov Report

Merging #1950 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1950   +/-   ##
=======================================
  Coverage   99.77%   99.77%           
=======================================
  Files         148      148           
  Lines        2637     2637           
=======================================
  Hits         2631     2631           
  Misses          6        6

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 69553ca...b0c41f4. Read the comment docs.

[levithomason]

Thanks

[levithomason]

Released in semantic-ui-react@0.71.4