Fixed several issues in the README

[Synthesid]

Hi! I'm currently trying to get a job as a tech writer and building a portfolio. I came upon your project and decided to help out as much as I currently can. I would hugely appreciate it if you won't mind me putting a link to your repo in my portfolio.
The rough changelog attached below. Cheers!

• Added Table of Contents
• Moved 'Sponsors' section down to group it with other community & ecosystem related sections
• Moved 'Building Fira Code locally' section further up from community & ecosystem related sections
• Replaced "Fira Code has fantastic support for ASCII/box drawing..." with "Fira Code has seamless support for ASCII/box drawing..."
• Replaced "In action:" with "Here's how it looks:"
• Replaced "Works" and "Doesn't work" in compatibility sections with "Compatible" and "Incompatible"
• Reorganized the end of the table in 'Editor compatibility list' section, replaced "Probably work" with "Likely compatible" and "Under Question" with "Compatibility status pending"
• Replaced "...this is the setup I use on macOS" with "...you can use my setup for macOS" for readability and consistency with the next paragraph
• Rewritten several paragraphs in 'Building Fira Code locally' section to align the text with Google tech writing standards
• Mostly eliminated passive voice throughout the document
• Edited minor oversights (missing spaces, commas, internally inconsistent punctuation, etc.)

[tonsky]

Thanks! I forgot to remove the sponsors. I might look at your suggestions later but I definitely don’t want all of them, so I’ll close this for now