Add status badges to GitHub README

[volcan01010]

Description

Following a suggestion from @meghanrjones, adding status badges to the README will "facilitate quick evaluation of the build/test status, test coverage, and PyPI project page."

See openjournals/joss-reviews#3649 (comment) for details.

Acceptance criteria

GitHub landing page README contains:

• Build status badge
• Test coverage badge
• PyPI version badge ()