[Doc] Add readme for C and C# directory (#1452)

add docs

csharp/README.md

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+# C# API implementation
+
+This directory is the implementation of FastDeploy C# SDK, which provides solutions for scenarios where users need C# API.
+
+## How to compile and install
+
+When compiling the fastdeploy library, enable the compilation option --WITH_CSHARPAPI=ON to enable compilation for this directory. The compiled C# library is in the csharp_lib directory after installation. For example, to compile the CPU version of the fastdeploy library under Windows and add the C# API library, you can use the following command
+
+```shell
+git clone https://github.com/PaddlePaddle/FastDeploy.git
+cd FastDeploy
+mkdir build && cd build
+cmake .. -G "Visual Studio 16 2019" -A x64 ^
+          -DENABLE_ORT_BACKEND=ON ^
+          -DENABLE_PADDLE_BACKEND=ON ^
+          -DENABLE_OPENVINO_BACKEND=ON ^
+          -DENABLE_VISION=ON ^
+          -DENABLE_TEXT=ON ^
+          -DWITH_CAPI=ON ^
+          -DWITH_CSHARPAPI=ON ^
+          -DCMAKE_INSTALL_PREFIX="D:\Paddle\compiled_fastdeploy"
+nuget restore #（please execute it when WITH_CSHARPAPI=ON to prepare dependencies in C#)
+msbuild fastdeploy.sln /m /p:Configuration=Release /p:Platform=x64
+msbuild INSTALL.vcxproj /m /p:Configuration=Release /p:Platform=x64
+```
+For more details about compiling the fastdeploy library, please refer to the documentation
+- [FastDeploy installation](../docs/cn/build_and_install/README.md)
+
+## how to use
+
+Use the compiled C# dynamic library file fastdeploy_csharp.dll as a reference to use the corresponding API. For usage examples, please refer to the use cases under examples
+
+- [paddleclas](../examples/vision/classification/paddleclas/csharp/README.md)
+- [paddledetection](../examples/vision/detection/paddledetection/csharp/README.md)
+- [pp-ocrv2](../examples/vision/ocr/PP-OCRv2/csharp/README.md)
+- [paddleseg](../examples/vision/segmentation/paddleseg/cpu-gpu/csharp/README.md)
+
+## Other documents
+
+- [How to add C# API to a new model](../docs/en/faq/develop_c_sharp_api_for_a_new_model.md)
+- [Vision Results](../docs/api/vision_results/README.md)

csharp/README_CN.md

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+# C# API实现
+
+该目录下为FastDeploy C# SDK的接口实现，为用户需要C# API的场景提供解决方案。
+
+## 如何编译安装
+
+在编译fastdeploy库的时候，打开编译选项 --WITH_CSHARPAPI=ON，即可开启对于该目录的编译。编译后的C#库安装后在csharp_lib目录下。例如在Windows下编译CPU版本的fastdeploy库并且加上C# API库可以使用如下命令
+
+```shell
+git clone https://github.com/PaddlePaddle/FastDeploy.git
+cd FastDeploy
+mkdir build && cd build
+cmake .. -G "Visual Studio 16 2019" -A x64 ^
+         -DENABLE_ORT_BACKEND=ON ^
+         -DENABLE_PADDLE_BACKEND=ON ^
+         -DENABLE_OPENVINO_BACKEND=ON ^
+         -DENABLE_VISION=ON ^
+         -DENABLE_TEXT=ON ^
+         -DWITH_CAPI=ON ^
+         -DWITH_CSHARPAPI=ON ^
+         -DCMAKE_INSTALL_PREFIX="D:\Paddle\compiled_fastdeploy"
+nuget restore  #（please execute it when WITH_CSHARPAPI=ON to prepare dependencies in C#)
+msbuild fastdeploy.sln /m /p:Configuration=Release /p:Platform=x64
+msbuild INSTALL.vcxproj /m /p:Configuration=Release /p:Platform=x64
+```
+关于编译fastdeploy库的更多详情信息可以参考文档
+- [FastDeploy安装](../docs/cn/build_and_install/README.md)
+
+## 如何使用
+
+将编译出来的C#动态库文件fastdeploy_csharp.dll作为引用即可以使用相应的API。关于使用示例可以参考examples下的用例
+
+- [paddleclas](../examples/vision/classification/paddleclas/csharp/README_CN.md)
+- [paddledetection](../examples/vision/detection/paddledetection/csharp/README_CN.md)
+- [pp-ocrv2](../examples/vision/ocr/PP-OCRv2/csharp/README_CN.md)
+- [paddleseg](../examples/vision/segmentation/paddleseg/cpu-gpu/csharp/README_CN.md)
+
+## 其它文档
+
+- [如何给新模型增加C# API](../docs/cn/faq/develop_c_sharp_api_for_a_new_model.md)
+- [Vision Results](../docs/api/vision_results/README_CN.md)