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

add docs

c_api/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, turn on the compile option --WITH_CAPI=ON to start compiling for this directory. The compiled C library and fastdeploy library are in the same dynamic library file, such as libfastdeploy.a under Linux. For example, to compile the CPU version of fastdeploy and compile the C API into the library, you can use the following command
+
+```bash
+git clone https://github.com/PaddlePaddle/FastDeploy.git
+cd FastDeploy
+mkdir build && cd build
+cmake .. -DENABLE_ORT_BACKEND=ON \
+          -DENABLE_PADDLE_BACKEND=ON \
+          -DENABLE_OPENVINO_BACKEND=ON \
+          -DCMAKE_INSTALL_PREFIX=${PWD}/compiled_fastdeploy_sdk \
+          -DENABLE_VISION=ON \
+          -DWITH_CAPI=ON \
+          -DOPENCV_DIRECTORY=/usr/lib/x86_64-linux-gnu/cmake/opencv4 \
+          -DENABLE_TEXT=ON
+make -j12
+make install
+```
+For more details about compiling the fastdeploy library, please refer to the documentation
+- [FastDeploy installation](../docs/en/build_and_install/README.md)
+
+## How to use
+
+ The header files of the provided C API can be used. If it is manually compiled and installed, such as the above command, after `make install`, the header files will be in the directory ${PWD}/compiled_fastdeploy_sdk/include/fastdeploy_capi/. If it is a downloaded precompiled library, after decompression,  include/fastdeploy_capi/ is the header file of the C API. For usage examples, please refer to the use cases under examples
+
+- [paddleclas](../examples/vision/classification/paddleclas/c/README.md)
+- [paddledetection](../examples/vision/detection/paddledetection/c/README.md)
+- [pp-ocrv2](../examples/vision/ocr/PP-OCRv2/c/README.md)
+- [paddleseg](../examples/vision/segmentation/paddleseg/cpu-gpu/c/README.md)
+
+## Other documents
+
+- [How to Develop C API for a New Model](../docs/en/faq/develop_c_api_for_a_new_model.md)
+- [Vision Results](../docs/api/vision_results/README.md)

c_api/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_CAPI=ON，即可开启对于该目录的编译。编译后的C库和fastdeploy库在同一个动态库文件中，如linux下为libfastdeploy.a。比如，编译CPU版本的fastdeploy并且将C API一同编入库可以使用如下命令
+
+```bash
+git clone https://github.com/PaddlePaddle/FastDeploy.git
+cd FastDeploy
+mkdir build && cd build
+cmake .. -DENABLE_ORT_BACKEND=ON \
+         -DENABLE_PADDLE_BACKEND=ON \
+         -DENABLE_OPENVINO_BACKEND=ON \
+         -DCMAKE_INSTALL_PREFIX=${PWD}/compiled_fastdeploy_sdk \
+         -DENABLE_VISION=ON \
+         -DWITH_CAPI=ON \
+         -DOPENCV_DIRECTORY=/usr/lib/x86_64-linux-gnu/cmake/opencv4 \
+         -DENABLE_TEXT=ON
+make -j12
+make install
+```
+关于编译fastdeploy库的更多详情信息可以参考文档
+- [FastDeploy安装](../docs/cn/build_and_install/README.md)
+
+## 如何使用
+
+因为提供的C API的头文件即可进行使用，如果是手动编译安装，比如上述命令，`make install`之后，头文件将会在目录${PWD}/compiled_fastdeploy_sdk/include/fastdeploy_capi/下。如果是下载的预编译好的库，解压缩之后include/fastdeploy_capi/下即是C API的头文件。关于使用示例可以参考examples下的用例
+
+- [paddleclas](../examples/vision/classification/paddleclas/c/README_CN.md)
+- [paddledetection](../examples/vision/detection/paddledetection/c/README_CN.md)
+- [pp-ocrv2](../examples/vision/ocr/PP-OCRv2/c/README_CN.md)
+- [paddleseg](../examples/vision/segmentation/paddleseg/cpu-gpu/c/README_CN.md)
+
+## 其它文档
+
+- [如何给新模型增加C API](../docs/cn/faq/develop_c_api_for_a_new_model.md)
+- [Vision Results](../docs/api/vision_results/README_CN.md)