Many models are missing README files

[GHTaarn]

According to the project README, all contributions should include a README. It would be very helpful for new users (including me) if this applied to all models, but none of the models that I have looked at have one. This includes

cppn dcgan_mnist mnist

[ToucheSir]

Those 3 and more were resolved in #305, but I think we can leave this issue around as a place to track which models/demos are still missing READMEs. If anyone comes across one, feel free to comment here and I'll append them as a checklist to this comment.