text: Title: Add a CI lint task to check st2client's README.md
  Body: We need to make sure that the st2client `README.rst` file is acceptable to PyPI, since any syntax errors in it will cause the `push_st2client` task of the `st2cd.st2_finalize_release` workflow to fail.
  
  We can check the syntax using the same renderer that PyPI itself uses:
  
  ```bash
  # Use the same README renderer that PyPI uses to catch syntax issues in the
  # README.rst file  # st2client uses README.rst
  # https://pypi.org/help/#description-content-type
  # https://pypi.org/project/readme-renderer
  # https://packaging.python.org/tutorials/packaging-projects/#description
  echo "Checking README.rst syntax"
  virtualenv venv-st2client-readme-checker
  . venv-st2client-readme-checker/bin/activate
  pip install --upgrade readme_renderer
  python -m readme_renderer README.rst
  deactivate
  ```
  
  It would be nice if we could catch these errors before release, which means that we should create a step in our CI tooling to check it before any bad changes get merged.