Add cdn info

[PeterDaveHello]

HI @geuis,

I'm Peter from cdnjs.com,

I wonder if you would like to add cdn info in the readme and github page?

This is the page we provide cdn for helium-css - https://cdnjs.com/libraries/helium-css

Thanks!

[geuis]

Updated the readme.

[PeterDaveHello]

Thank you.