Docs/pypi install info in readme

[leifdenby]

Describe your changes

Two changes to the README:

• change instructions for uvx based command-line execution of validator to use python package directly from pypi (rather than getting sourcecode from github) now that mlcast-dataset-validator is released on pypi
• add usage example for the python api that will be used in mlcast-datasets

The motivation here is to 1) ensure that people by default only validate datasets with releases of the validator published to pypi, and 2) make people aware that the validator also has a python api.

No changes of dependencies required for this change.

Issue Link

Partially completes #2

Type of change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality, e.g. adding a new dataset)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected, e.g. removing or moving a dataset in the catalog)
• 📖 Documentation (Addition or improvements to documentation)
• 🔨 Maintenance (Fixing issues with CI/CD and changes to upstream dependencies)

Checklist before requesting a review

• My branch is up-to-date with the target branch - if not update your fork with the changes from the target branch (use pull with --rebase option if possible).
• I have performed a self-review of my code
• For any new/modified functions/classes I have added docstrings that clearly describe its purpose, expected inputs and returned values
• I have placed in-line comments to clarify the intent of any hard-to-understand passages of my code
• I have updated the documentation to cover introduced code changes
• I have added tests that prove my fix is effective or that my feature works
• I have given the PR a name that clearly describes the change, written in imperative form (context).
• I have requested a reviewer and an assignee (assignee is responsible for merging)

Checklist for reviewers

Each PR comes with its own improvements and flaws. The reviewer should check the following:

• the code is readable
• the code is well tested
• the code is documented (including return types and parameters)
• the code is easy to maintain

Author checklist after completed review

• I have added a line to the CHANGELOG describing this change, in a section
  reflecting type of change (add section where missing):
  • added: when you have added new functionality
  • changed: when default behaviour of the code has been changed
  • fixes: when your contribution fixes a bug

Checklist for assignee

• PR is up to date with the base branch
• the tests pass
• author has added an entry to the changelog (and designated the change as added, changed or fixed)
• Once the PR is ready to be merged, squash commits and merge the PR.

[franchg]

lgtm!