Revise contribution instructions for OpenCV modules

Updated contribution guidelines for adding new OpenCV modules and documentation.

Author: rainyl

CONTRIBUTING.md

@@ -11,13 +11,18 @@
 
 If you want to add new opencv API/Module,
 
-1. build [libdartcv](https://github.com/rainyl/dartcv/blob/main/README.md) from source
-2. add new C wrappers in `src` and make sure it is correctly build
-3. add your `.h` file to ffigen in [dartcv4](https://github.com/rainyl/opencv_dart/tree/main/packages/dartcv) and run `dart run ffigen --config ffigen.yaml`
-4. add corresponding dart code in [lib](https://github.com/rainyl/opencv_dart/tree/main/packages/dartcv/lib)
-5. write testing code and test whether it works as expected
-
-Note: C wrappers should be added to [dartcv](https://github.com/rainyl/dartcv) and dart bindings should be added to [dartcv4](https://github.com/rainyl/opencv_dart/tree/main/packages/dartcv)
+1. take a look at OpenCV official documents and make sure the required functions/modules are absent in opencv_dart
+2. the native C/C++ code is managed by CMake, you will find a `CMakeLists.txt` in `packages/dartcv/src`
+3. add new C wrappers in `packages/dartcv/src/dartcv` and make sure it is correctly build, Note: 
+  - take a look at the existed code, especially `packages/dartcv/src/dartcv/core/types.h` which defines basic structures
+  - if you want to add new modules, create a new directory in `packages/dartcv/src/dartcv`, e.g., `packages/dartcv/src/dartcv/cuda`
+  - nearly all C-wrappers of C++ class/struct are wrapped in a struct, you can define a C wrapper of a C++ class via `CVD_TYPEDEF`, e.g., `CVD_TYPEDEF(cv::Mat, Mat)`
+4. add your `.h` file to ffigen in [dartcv4](https://github.com/rainyl/opencv_dart/tree/main/packages/dartcv/ffigen) and run `dart run ffigen --config ffigen.yaml`， this will generate corresponding dart:ffi wrappers.
+5. add corresponding dart code in [lib](https://github.com/rainyl/opencv_dart/tree/main/packages/dartcv/lib)
+6. write testing code and test whether it works as expected, you can refer to [OpencvSharp](https://github.com/shimat/opencvsharp) and [gocv](https://github.com/hybridgroup/gocv) to write tests. 
+
+~~Note: C wrappers should be added to [dartcv](https://github.com/rainyl/dartcv) and dart bindings should be added to [dartcv4](https://github.com/rainyl/opencv_dart/tree/main/packages/dartcv)~~
+Since `2.x`, C/C++ code are embeded to this (opencv_dart) repo and no longer needed to develop separetely, just implement your C/C++ code in `packages/dartcv/src/dartcv` and it will be processed by dart native hooks.
 
 ## Contribute to Documentation
 
@@ -31,9 +36,8 @@ For Windows:
 
 1. `git clone https://github.com/rainyl/opencv_dart`
 2. cd `opencv_dart/packages/dartcv`
-3. install dependencies and add dynamic library path to PATH environment variable.
+3. install dependencies via `dart pub get` ~~and add dynamic library path to PATH environment variable.~~
 4. now write new dart tests and place them in `test/` directory.
 5. run `dart test`
-6. (Optional) add the **absolute** path of opencv_dart/windows to PATH environment variable, which can ebable the testing sidebar of VSCode.
 
 Other platforms are similar.