Document how to install fonts for Sketch (#2993)

README.md

@@ -345,6 +345,10 @@ be sure to clear your cookies.
 
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+### Working with UX Mocks
+
+When implementing user interfaces you will need to refer to the [Sketch](https://www.sketchapp.com/) mocks that are located in the [assets](https://github.com/mozilla/addons-frontend/tree/master/assets) directory. You will need a license to run Sketch and you also need to install some fonts (which are free). Install [Fira Sans](https://www.fontsquirrel.com/fonts/fira-sans) and [Chivo](https://www.fontsquirrel.com/fonts/chivo).
+
 ## What version is deployed?
 
 You can check to see what commit of `addons-frontend` is deployed by