Point to alternatives in the README

[akaihola]

Since this project hasn't been updated in five years and isn't published on the PyPI, maybe it's good to mention more recent tools as alternatives in the README.

For example:

• pydoclint together with pydocstyle or ruff
• Pylint's docparams extension
• flake8-docstrings-complete
• pydoctest
• docsig
• darglint2 (often prohibitively slow though)

Also related:

• flake8-rst-docstrings checks ReStructuredText format in docstrings
• flake8-rst runs flake8 on code snippets in docstrings
• docstring-utils and docstring_parser are libraries which implement simple parsers for Numpy, Sphinx and Google-style docstrings.