Update Tailwind install guide

[AntonNiklasson]

Summary

I was going through the Tailwind setup guide and realised there's a few key things missing. Commands listed differes slightly from Tailwind's own guide for integrating it with Gatsby.

Looks like autoprefixer is required for npx tailwindcss init to succeed.

I'm open for PR'ing this if it looks like something that should be fixed.

[LekoArts]

Sure, @AntonNiklasson go ahead with a PR aligning our docs to their docs :)

[AntonNiklasson]

Cool, I'll get on this during the week.