Add instructions for installing/using on AWS Lambda

[pythonwebcoder]

I want to use face_recognition with cnn on AWS Lambda to vastly speed up the detection process in video.

[ageitgey]

This would be great to have, but I'm not currently not using Lambda much so I'm not in a great position to do it. I know others have tried and it can be a bit tricky because of the code size limits on Lambda with the size of the libraries/pre-requisites this requires.

In any case, PRs are welcome!