improve readme in a few ways

[danielfromearth]

(As part of ncompare's PyOpenSci review (issue #146))

• clarify poetry vs non-poetry in readme
  • "The tests can indeed run with poetry run pytest tests but pytest tests from a python venv works too, it could be nice to have this clarified in the README as both installations are previously described."
• license
  • "The license is partially available in the README and as pdf in the license directory: as this is a specific NASA license and probably less known by the users, I suggest to have its name specified in the README perhaps with its link at the OSI website, for better discoverability."
• Add readme badges for
  • Repostatus
  • Supported python versions
  • References:
    • https://www.repostatus.org/
    • Add badges to your README to show supported Python versions + software license ResidentMario/missingno#49 (comment)