Update README file

[sdwheeler]

This PR updates the README file in the following ways:

• Follow Microsoft style and formatting for documentation
  • Fix markdown formatting to be compliant with markdownlint
  • Fixed ATX header levels
  • Reflow paragraphs to 100-character line length limit for readability and maintainability
  • Change hyperlinks to markdown link references to improve line lengths and maintainability
• Changes links to point to the official documentation for Invoke-ScriptAnalyzer
• Improved grammar and wording for clarity
• Fixes indentation of YAML examples
• Add link to SARIF information
• Removed unneeded template instructions meant to be removed by the maintainer

[RogueScholar]

You've done yeoman's work here that's sorely needed. I only see a few places that could be improved with some more-natural sounding language. In all other places, it's rock-solid; nicely done. 👍🏻

[sdwheeler]

@A-Katopodis Could you review and merge?