Broken Link to "Learning Representations for Counterfactual Inference" Topic in README.md

[ppirch]

The link to the topic "Learning Representations for Counterfactual Inference" in the README.md file is currently broken. The original link no longer works.

I suggest updating the link in the README.md file to the following arXiv URL: https://arxiv.org/pdf/1605.03661. This link points to the PDF file containing the topic "Learning Representations for Counterfactual Inference."

[rpryzant]

Thanks! Feel free to submit a pull request :)