add contributors md and move dev docs out

CONTRIBUTORS.md

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+# Contributors (alphabetically)
+
+* **[madisonmay](https://github.com/madisonmay)**
+
+  Added Dockerfiles
+
+* **[Margaret Mitchell et al](https://arxiv.org/abs/1810.03993)**
+
+  Our [usage](./readme#usage) writeup was loosely inspired by the paper
+  [Model Cards for Model Reporting](https://arxiv.org/abs/1810.03993)
+  and related conversations with some of the authors.
+
+* **[webproduktion01](https://github.com/webproduktion01)**
+
+  Ported download script to python.
+
+**[Full code contributors list](https://github.com/openai/gpt-2/contributors).**