Improve README introduction

[ajhawkings]

These are a few edits that I think make the introduction clearer - let me know if you disagree and I can revert any of them:

• Remove manually created Table of Contents and explain how to access the auto-generated ones on GitHub and the live site
• Change the usage section to include finding and using badges (incorporating the previous tips section)
• Improve grammar in some lines
• Make sure that there is a newline after each heading to be consistent
• Replace the back to top link that previously linked to the table of contents with a new anchor link

Rendered preview of the changes: https://github.com/ajhawkings/markdown-badges/tree/introduction-improvement#top

[ajhawkings]

These are substantial changes so do let me know if you don't like them and I can change them/close the PR

[kohasummons]

Hey @ajhawkings. These contributions are great. Would you love to make a new pull request with the changes—just to avoid merge conflicts. Also, I think the TOC improves the navigation overall. I'd love to hear your thoughts on why it should be scrapped.