Add run on repl.it badge to README

This pull request adds a  badge to the . This will allow users to easily run this repository in their browser, without having to set up an environment. You can learn more about Repl.it [here](https://repl.it).

.replit

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+language = "python3"
+run = "<p><a href=\"https://github.com/affinelayer/pix2pix-tensorflow\"> [Tensorflow]</a> (by Christopher Hesse), <a href=\"https://github.com/Eyyub/tensorflow-pix2pix\">[Tensorflow]</a> (by Eyyüb Sariu), <a href=\"https://github.com/datitran/face2face-demo\"> [Tensorflow (face2face)]</a> (by Dat Tran), <a href=\"https://github.com/awjuliani/Pix2Pix-Film\"> [Tensorflow (film)]</a> (by Arthur Juliani), <a href=\"https://github.com/kaonashi-tyc/zi2zi\">[Tensorflow (zi2zi)]</a> (by Yuchen Tian), <a href=\"https://github.com/pfnet-research/chainer-pix2pix\">[Chainer]</a> (by mattya), <a href=\"https://github.com/tjwei/GANotebooks\">[tf/torch/keras/lasagne]</a> (by tjwei), <a href=\"https://github.com/taey16/pix2pixBEGAN.pytorch\">[Pytorch]</a> (by taey16) </p> </ul>"

README.md

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+[![Run on Repl.it](https://repl.it/badge/github/junyanz/pytorch-CycleGAN-and-pix2pix)](https://repl.it/github/junyanz/pytorch-CycleGAN-and-pix2pix)
 <img src='imgs/horse2zebra.gif' align="right" width=384>
 
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