README Cleanup

[aromanielloNTIA]

• Fixed a couple typos
• Added language annotations to code blocks for syntax highlighting when displayed on GitHub
• Fixed broken hyperlink formatting
• Formatted names of variables/classes referenced in the documentation using code style
• Replaced some HTML with native markdown (lists)
• Added top level heading with repo name "NTIA/ITS Preselector API"
• Moved some text around (from the Introduction section to under the main header)

The diff will look atrocious due to changes in line breaks rather than actual changes to content

[aromanielloNTIA]

For review it is probably easier to see the changes by looking at the rendered markdown here instead of the diff.