Document parameters in README.md

We've created a few extra parameters to change the behavior of the page. For example,

seo_title: "" headline: "" subtitle: "" description: "" tagline: "" links: [[href: "url", text: "text"], [href: "url", text: "text"], [href: "url, text: "text"]]

We should be documenting them all in the readme.md

We should do the same for parameters in config.toml file.