Documentation: command-line options

[shimizukawa]

Currently we can only know command-line options by the command line --help option.
Issues #79 and #85 are examples that could have been solved with documentation.

[shimizukawa]

merged #93 and published.

• build process: https://readthedocs.org/projects/sphinx-intl/builds/21950993/
• build result: https://sphinx-intl.readthedocs.io/en/master/refs.html#sphinx-intl