Like many of you, I bought a license for Font Awesome 5 during the Kickstarter years ago. Also like many of you, I could never have figure out how to use FA5 in my RW projects. Thanks to some support by Nick Cates on an unrelated question, I now know how! And you can, too!
Go to: https://fontawesome.com/start
Sign-in to your account, and create a Kit. FA will give you a <script>. Copy that, and put it in the Head Global Settings of your RW project.
Now, you can put in <i class> tags of FA5 icons wherever you like.
RW won’t display the icons in edit mode. If I understand it correctly, FA changed the licensing with v5, which made it tough for Stacks to license it. However, in preview mode and when published, it shows up great.
Enjoy FA5!