Add CONTRIBUTORS.md

CONTRIBUTORS.md

+# Contributors
+
+This is a list of people who contributed patches to keras-retinanet.
+
+If you feel you should be listed here or if you have any other questions/comments on your listing here,
+please create an issue or pull request at https://github.com/fizyr/keras-retinanet/
+
+* Hans Gaiser <h.gaiser@fizyr.com>
+* Maarten de Vries <maarten@de-vri.es>
+* Ashley Williamson
+* Yann Henon
+* Sorin Panduru
+* Enrico Liscio <e.liscio@fizyr.com>
+* Mihai Morariu
+* Wudi Fang
+* yhenon
+* Mike Clark
+* Valeriu Lacatusu
+* mxvs
+* mwilder
+* hnsywangxin