1. Thank you! 2. little documentation improvement

[afkrause]

Dear Håkon, first of all i want to thank you for providing this very useful and easy to use python module.
Next, i have a small suggestion how to improve the documentation / README.md:
please document the available options for build_detector(...). I had to look into the source code (ok, always a good idea) to figure out what the string for retina face is ("RetinaNetResNet50").
Would be nice if that would be placed upfront into the README.

Thanks again.