[IR] Update README.md (#1445)

Add a bullet point in the features around protobuf.

Author: justinchuby

onnxscript/ir/README.md

@@ -10,6 +10,7 @@ An in-memory IR that supports the full ONNX spec, designed for graph constructio
 - Robust mutation: Create as many iterators as you like on the graph while mutating it.
 - Speed: Performant graph manipulation, serialization/deserialization to Protobuf.
 - Pythonic and familiar APIs: Classes define Pythonic apis and still map to ONNX protobuf concepts in an intuitive way.
+- No protobuf dependency: The IR does not require protobuf once the model is converted to the IR representation, decoupling from the serialization format.
 
 ## Code Organization 🗺️