Fix typos and formatting issues in the README, also add a TOC.

[jxmot]

• Add a TOC
• For "oghead-example-test.json" remove line with "#"
• Change heading from "Additional Open Graph Information" to "SEO and Open Graph"
• In "LinkedIn Notes" fix link at end of paragraph

[jxmot]

Had attempted to enclose the TOC in a details/summary tag. But would only work parsed through GitHub. The could here would not render it correctly.