add opencv usage doc

examples/camera_opencv/README.md

@@ -0,0 +1,23 @@
+OpenCV Usage
+=====
+
+This is an example of using OpenCV with libmaix.
+
+## Build
+
+See [README.md](../../README.md) for build instructions and compile `examples/hello-world` first.
+
+Then build this example:
+
+```
+python project.py --toolchain /opt//opt/toolchain-sunxi-musl/toolchain/bin --toolchain-prefix arm-openwrt-linux-muslgnueabi- config
+python project.py build
+```
+
+## imlib (OpenMV libs)
+
+If you want to use OpenMV's lib like `find_blobs`, you can refer to [this API list](https://github.com/sipeed/MaixPy3/blob/c6b5c419a9c547f1f42c686020eb0e4cdb3f93cf/ext_modules/_maix_image/py_maix_image.cpp#L105) to find what you want, then find the corresponded C function, e.g. `.def("binary", &maix_image::_binary,` point to [_binary](https://github.com/sipeed/MaixPy3/blob/c6b5c419a9c547f1f42c686020eb0e4cdb3f93cf/ext_modules/_maix_image/_maix_image.cpp#L926) function, then you use this `binary` method in your C code!
+
+
+
+