Documentation: fix readme linting errors

[qdm12]

Linted with Vscode's davidanson.vscode-markdownlint (based on https://github.com/DavidAnson/markdownlint)

• Use consistent bullet points * instead of mixed * and -
• Insert new lines where appropriate
• Use consistent heading levels
• Put title at top of readme before badges
• Add language sh (shell) for syntax highlighting of code blocks

This blocks 2 PRs I want to do:

• add an amd64 Docker (itself blocking a cross cpu docker) with instructions in readme
• complete/update documentation for setup on Debian systems