feat(starters): Add font smoothing

[jahlherapath]

This PR adds font smoothing via layout.css to the default gatsby starter theme.

[DSchau]

Cool! So few questions/comments:

1. Could you explain in a sentence or two why this is valuable?
   • In reading, looks like it makes text look a little nicer (yay); does this have any ill-effect on Windows vs. Mac?
2. If this is worth doing (and it probably is!) may as well add it to all starters?

[jahlherapath]

Thanks for the feedback! @DSchau

1. Yeah it basically makes the text more legible and a good inclusion for sites with heaps of content. No issues between platforms.
2. I could add to the other starters but there are no default styles (mostly inline).

[wardpeet]

@DSchau windows will just ignore it as they don't support font smoothing. A workaround is using SVGs for windows but it can gives you weird issues as it's less flexible as woff, ...

By adding this it will only make osx a bit smoother and I'm all for it! 😄

[DSchau]

Works for me! Thanks!

[gatsbot]

Holy buckets, @jahlherapath — we just merged your PR to Gatsby! 💪💜

Gatsby is built by awesome people like you. Let us say “thanks” in two ways:

1. We’d like to send you some Gatsby swag. As a token of our appreciation, you can go to the Gatsby Swag Store and log in with your GitHub account to get a coupon code good for one free piece of swag. (Currently we’ve got a couple t-shirts available, plus some socks that are really razzing our berries right now.)
2. We just invited you to join the Gatsby organization on GitHub. This will add you to our team of maintainers. Accept the invite by visiting https://github.com/orgs/gatsbyjs/invitation. By joining the team, you’ll be able to label issues, review pull requests, and merge approved pull requests.

If there’s anything we can do to help, please don’t hesitate to reach out to us: tweet at @gatsbyjs and we’ll come a-runnin’.

Thanks again!