Add link to the Stylelint guide

[simurai]

This adds a link to the Stylelint guide added in https://github.com/github/design-systems/pull/735.

/cc @primer/ds-core

[vercel]

This pull request is being automatically deployed with ZEIT Now (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://zeit.co/primer/primer-css/8tuw3551r
🌍 Preview: https://primer-css-git-stylelint-guide.primer.now.sh

[jonrohan]

I wonder if we should pull the whole stylelint file into primer.style instead of linking to it?

It seems like good content to show off to other teams. ⚡️ 😎

[simurai]

I wonder if we should pull the whole stylelint file into primer.style instead of linking to it?

Hmm.. Was thinking about that too. But it feels a bit odd talking about scripts used internally for another repo (github/github). If we wanted to show off about it, maybe writing a blog post or doing a talk with demos would be more helpful?

Anyways, I'll merge this for now. It's more meant for Hubbers to point them in the right direction.