Add "ONNX Ops" to README.

README.md

@@ -5,3 +5,15 @@ Test suites for IREE and related projects.
 🚧🚧🚧 Under construction 🚧🚧🚧
 
 See https://groups.google.com/g/iree-discuss/c/GIWyj8hmP0k/ for context.
+
+## Test suites
+
+### ONNX Ops ([onnx-ops/](onnx-ops/))
+
+[![Test ONNX Ops](https://github.com/iree-org/iree-test-suites/actions/workflows/test_onnx_ops.yml/badge.svg?branch=main&event=push)](https://github.com/iree-org/iree-test-suites/actions/workflows/test_onnx_ops.yml?query=event%3Apush+branch%3Amain)
+
+* 1250+ tests for ONNX (Open Neural Network Exchange: https://onnx.ai/)
+  operators (https://onnx.ai/onnx/operators/).
+* Runnable via [pytest](https://docs.pytest.org/en/stable/) using a
+  configurable set of flags to `iree-compile` and `iree-run-module`, allowing
+  for testing across different compilation targets and runtime devices.