Add cronstrue binary

[andersjanmyr]

$ npm install -g cronstrue

$ cronstrue 1 2 3 4 5
At 02:01 AM, on day 3 of the month, and on Friday, only in April

$ cronstrue 1 2 3
Error: too few arguments (3): 1 2 3
Usage (5 args): cronstrue minute hour day-of-month month day-of-week
or
Usage (6 args): cronstrue second minute hour day-of-month month day-of-week
or
Usage (7 args): cronstrue second minute hour day-of-month month day-of-week year

[bradymholt]

Nice! Would you be up for adding a subsection to Usage in the README explaining the CLI usage?