Add link to C API doc (#3375)

See doc at https://k2-fsa.github.io/sherpa/onnx/c-api/html/index.html

Author: csukuangfj

sherpa-onnx/c-api/README.md

@@ -1,13 +1,18 @@
 # Introduction
 
 
+## View doc
+
+You can find documentation for C API and CXX API at the following address:
+<https://k2-fsa.github.io/sherpa/onnx/c-api/html/index.html>
+
 ## Generate doc
 
-```
+```bash
 sudo apt install doxygen graphviz      # Ubuntu/Debian
 brew install doxygen graphviz          # macOS
 ```
 
-```
+```bash
 doxygen ./Doxyfile
 ```