Highlight the resources and writing style guides more

[rmglennon]

I've done some work on the writing style guide, so let's find a way to highlight it more in the style guide (or move it elsewhere if it doesn't belong here).

Right now, the writing guidelines are linked to md files (some are broken, though) right in GitHub
https://mapzen.com/common/styleguide/resources.html

As the writing style content grows, I'd like to see it more prominent/in the TOC.