Comment that data of THStorage may be NULL.

Signed-off-by: Edward Z. Yang <[email protected]>

Author: ezyang

torch/csrc/README.md

@@ -30,6 +30,11 @@ One important case where this assumption is important is when tracking
 the CUDA device a tensor is stored in: this information is stored
 solely in the storage, so if a storage is NULL, we lose this information.
 
+Although storage is never NULL, the data field of THStorage may be NULL.  This
+mostly occurs when we want to pre-allocate an output tensor struct, but then
+have it be resized and filled with data by some operator: there's no point in
+allocating data for it in this case!
+
 ## Files
 
 ### `Exceptions.h`