Update README

[Insprill]

This makes a few changes to the README.md

• Fixed various grammar issues.
• Adjusted header sizes to make it easier to find specific sections.
• Added links to the unofficial AUR and Homebrew packages (Closes Added macOS brew installation #314).
• Updated the MSRV.
• Broke up long single lines into multiple lines. They'll still render the same, but be easier to edit in the future.
• Moved platform-specific building requirements into their own dropdowns.

[jacksongoode]

This looks great, should we also update the roadmap? Or perhaps include it as a separate roadmap.md file we link to?

[Insprill]

I think leaving the roadmap in the readme is fine, as it makes it easy for people to see a high-level overview of Psst's features. What on it do you think should be updated?

[jpochyla]

Nice, thanks!