message: Add an extensibility section to IR.md
diff: @@ -36,7 +36,6 @@ The following are the metadata properties of a model graph:
  |ir_version|int64||The version of the IR format specification|
  |doc_string|string|Free form|A human-readable documentation string intended to summarize the purpose of the model. Markdown is allowed.|
  -
  #### Names Within a Graph
  We organize names into separate namespaces. Names must be unique within a namespace.
  @@ -82,3 +81,22 @@ The following data types are supported by ONNX. Additional data types can be sup
  |Other|__bool__|Boolean value represent data with only two values, typically _true_ and _false_.|
  |Other|__handle__|Handles are opaque types holding a 64-bit integer.|
  |Collections|__sparse and dense tensor__|Tensors are a generalization of vectors and matrices; whereas vectors have one dimension, and matrices two, tensors can have any number of dimensions, including zero. A zero-dimensional tensor is equivalent to a scalar.|
  +
  +Extensibility
  +-------------
  +
  +ONNX is expected to evolve over time and provides features that allow users to experiment and implement additions before they are folded into the specifications.
  +
  +### Metadata
  +
  +A model allows named metadata strings to be added via its `metadata_props` field, typically for use by tools such as converters, indexers or documentation generators. Names are not prescribed, but some name recommendations are provided for implementations that want to record such concepts.
  +
  +- `author`: name of the person(s) who authored the model
  +- `company`: name of the company or organization that authored the model
  +- `converted_from`: if converted from a different format, the name of the source format or framework
  +- `license`: a human-readable name for a license if applicable
  +- `license_url`: the URL where the license text is provided
  +
  +### Operators
  +
  +Operators may be extended via custom domain names in the `opset_import` field.
  