Mention Fontconfig when requiring Cairo
Mention Fontconfig when requiring Cairo

Fontconfig is necessary to make sure that the text sizes are calculated
correctly. Hence, when a user installs Cairo, they should also install
Fontconfig.

dcjones/Compose.jl,Hydrotoast/Compose.jl,jiahao/Compose.jl,JuliaPackageMirrors/Compose.jl