added formula

torch_scatter/functions/__init__.py

-import torch
-
 from .scatter import scatter
 from .utils import gen_filled_tensor, gen_output
 
@@ -18,6 +16,10 @@ def scatter_add_(output, index, input, dim=0):
     size :math:`(x_0, ..., x_{i-1}, y, x_{i+1}, ..., x_{n-1})`. Moreover, the
     values of :attr:`index` must be between `0` and `output.size(dim) - 1`.
 
+    For one-dimensional tensors, the formula is given by
+    :math:`output_i = \sum_j input_j` where sum is over :math:`j` such that
+    :math:`index_j = i`.
+
     Args:
         output (Tensor): The destination tensor
         index (LongTensor): The indices of elements to scatter