Make some improvements to the readme

[jtraglia]

This PR fixes some random nits I had with the readme:

• Add badges for python package version, license.
• Use prettier discord badge & put it last.
• Uncapitalize words after the first word in sections, for consistency.
• Rename "in-development" to "unstable" for simplicity & consistency.
• Remove unnecessary paragraph about additional specs.
• Convert sentence about reference tests into bulleted list.
• Use SSH link in cloning instructions.
• Add license to pyproject.toml (to go with our new license badge).

Note: The license badge will not work until our next release.