Document that feature files need to be specified
Document that feature files need to be specified

Previously running `gherkin-lint -h` printed this:

    Usage: gherkin-lint [options]

This made me think that the linter somehow searches for
the *.feature files by itself.  Took me a while to figure out
that they actually need to be specified from command line.
After this change, the usage info is:

    Usage: gherkin-lint [options] <feature-files>

vsiakka/gherkin-lint