Add note about running npm install on MacOS

[nicolo-ribaudo]

Running npm install on MacOS requires first installing some native libraries, so that canvas can be properly built:

brew install pkg-config cairo pango libpng jpeg giflib librsvg pixman

This PR adds a link to canvas' building instructions, so that other people don't have to spend too much time trying to figure out why npm install is failing 🙂

[timvandermeij]

Looks good; thank you for improving this!