Add rst-lint back to the lint utilities.

[rjdbcm]

The lint checkpoint, and checkpoints in general, are to check that a project can be packaged, released, and published. Twine will fail to upload bad readme text and we should not be letting this happen.

This can't catch every issue see https://pypi.org/project/restructuredtext-lint/:

While a document may lint cleanly locally, there can be issues when submitted it to [PyPI](http://pypi.python.org/). Here are some common problems:

    Usage of non-builtin lexers (e.g. bibtex) will pass locally but not be recognized/parsable on [PyPI](http://pypi.python.org/)

        This is due to [PyPI](http://pypi.python.org/) not having a non-builtin lexer installed

        Please avoid non-builtin lexers to avoid complications

        For more information, see [#27](https://github.com/twolfson/restructuredtext-lint/issues/27)

    Relative hyperlinks will not work (e.g. ./UNLICENSE)

        According to Stack Overflow, hyperlinks must use a scheme (e.g. http, https) and that scheme must be whitelisted

            http://stackoverflow.com/a/16594755

        Please use absolute hyperlinks (e.g. https://github.com/twolfson/restructuredtext-lint/blob/master/UNLICENSE)

However, these are fair restrictions to have on a shared README file.