Add a comment explaining use of subtract for speed

Author: ehgus

numcodecs/delta.py

@@ -64,6 +64,7 @@ def encode(self, buf):
         enc[0] = arr[0]
 
         # compute differences
+        # using np.subtract for in-place operations
         np.subtract(arr[1:], arr[0:-1], out=enc[1:])
 
         return enc