Update README about underscore font specifications of e.g FontAwesome5_Brands etc.

[jayjo34]

Hi and thank you for your effort keeping this project awesome!

I think it would be a huge help if you could add the information about the font specifications for those which need underscore, e.g. FontAwesome5_Brands.
Every time I need to implement FontAwesome I'm stuck at this point, because i forgot to implement those underscore libs and do something like :
project.ext.vectoricons = [ iconFontNames: [ 'FontAwesome5.ttf' ] ]
😑

Thanks and happy Xmas !