Add scripts to export ASR models from wenet to ONNX (#425)

See
https://user-images.githubusercontent.com/5284924/282995968-f6d39118-8008-4ce7-9d7c-d1d6387ac183.png

scripts/wenet/README.md

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+# Introduction
+
+This folder contains script for exporting models
+from [wenet](https://github.com/wenet-e2e/wenet)
+to onnx. You can use the exported models in sherpa-onnx.
+
+Note that both **streaming** and **non-streaming** models are supported.
+
+We only use the CTC branch. Rescore with the attention decoder
+is not supported, though decoding with H, HL, and HLG is supported.