added comment to DropMask

mhcflurry/keras_layers/drop_mask.py

 from keras.layers import Layer
 
 class DropMask(Layer):
+    """
+    Sometimes we know that a mask is always going to contain 1s (and never 0s)
+    due to e.g. slicing the beginning of a sequence with a known min length.
+    In that case it can be useful to drop the sequence mask and feed the
+    activations to a layer which does not support masking (e.g. Dense).
+    """
     supports_masking = True
 
     def call(self, x, mask):