message: Add section about external tensor data to IR.md
  Support for tensor data to be stored in external files was added in
  PR Add minimal documentation to the IR spec. Fixes
diff: @@ -291,6 +291,9 @@ There are two ways to leave an optional input or output unspecified: the first,
  Each node referring to an operator with optional outputs MUST provide a name for each output that is computed and MUST NOT provide names for outputs that are not computed.
  +#### External Tensor Data
  +
  +The raw data for large constant tensors, such as initializers, MAY be serialised in a separate file. In such a case, the tensor MUST provide the filename relative to the model file and MUST NOT use the value fields. It MAY provide a byte offset and length within that file. It MAY also specify a SHA1 digest of the file. One file MAY contain the data for multiple tensors.
  ## Standard data types
  