Add 5-paragraph summary of the repository content.

[markbpryan]

This commit introduces a new file, SUMMARY.md, which contains a 5-paragraph summary of the book "The Markdown Guide". The summary outlines the book's purpose, covers its main sections including introductory concepts, basic syntax, extended syntax, and mentions the final cheat sheet. This provides a quick overview of the repository's contents for you.

[markbpryan]

test 2