Comment about settings workers=0 on Windows.

model.py

@@ -2189,6 +2189,9 @@ class MaskRCNN():
         self.set_trainable(layers)
         self.compile(learning_rate, self.config.LEARNING_MOMENTUM)
 
+        # Work-around for Windows: Keras fails on Windows when using
+        # multiprocessing workers. See discussion here:
+        # https://github.com/matterport/Mask_RCNN/issues/13#issuecomment-353124009
         if os.name is 'nt':
             workers = 0
         else: