Add a reminder to install dependencies first

[tillsanders]

Our repositories are on GitLab, grouped in several levels of subgroups. So to collect licensing information, I first cloned all of them using ghorg. By the way, you might want to mention that tool because it really compliments npm-license-crawler for use cases like mine. So I now have dozens of repositories and running npm-license-crawler on them. However, I forgot to install of the dependencies first to get any feedback. A rather obvious mistake, but it still took me a few minutes to figure out. Maybe it would be nice to add a hint to the docs :)

Now it works great – thank you for your work!