How can you contribute?
If you know any geometry you probably know more than me, so there are plenty of improvements to algorithms waiting to be made. There are some TODO comments sprinkled through the code, but plenty more opportunities for improvement.
Update the code, most of which was written for Julia versions 0.2, v0.3 and 0.4 (remember when broadcasting wasn't a thing?) so there are probably many areas where the code could take more advantage of version 1.
There can always be more tests, as the present tests are mainly visual, showing that something works, but there should be much more testing of things that shouldn't work - inappropriate input, overlapping polygons, coincident or collinear points, anticlockwise polygons, etc.
More systematic error-handling particularly of geometry errors would be a good idea, rather than sprinkling
throw(error())s around when things look wrong.