docs(bindings/C): C binding contributing documentation

bindings/c/CONTRIBUTING.md

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+# Contributing
+1. [Contributing](#contributing)
+   1. [Setup](#setup)
+      1. [Using a dev container environment](#using-a-dev-container-environment)
+      2. [Bring your own toolbox](#bring-your-own-toolbox)
+   2. [Build](#build)
+   3. [Test](#test)
+   4. [Docs](#docs)
+   5. [Misc](#misc)
+
+## Setup
+
+### Using a dev container environment
+OpenDAL provides a pre-configured [dev container](https://containers.dev/) that could be used in [Github Codespaces](https://github.com/features/codespaces), [VSCode](https://code.visualstudio.com/), [JetBrains](https://www.jetbrains.com/remote-development/gateway/), [JuptyerLab](https://jupyterlab.readthedocs.io/en/stable/). Please pick up your favourite runtime environment.
+
+The fastest way is:
+
+[![Open in GitHub Codespaces](https://github.com/codespaces/badge.svg)](https://codespaces.new/apache/incubator-opendal?quickstart=1&machine=standardLinux32gb)
+
+### Bring your own toolbox
+To build OpenDAL C binding, the tools you need are very simple.
+
+The following is all you need
+- **A C++ compiler** that supports **c++14**, *e.g.* clang++ and g++
+- **A Rust toolchain** that satisfies the MSRV
+- **(optional)** if you want to format the code, you need to install **clang-format**
+    - The `opendal.h` is not formatted by hands when you contribute, please do not format the file. **Use `make format` only.**
+    - If your contribution is related to the files under `./tests`, you may format it before submitting your pull request. But notice that different versions of `clang-format` may format the files differently.
+- **(optional)** **GTest(Google Test)** need to be installed if you want to build the BDD (Behavior
+  Driven Development) tests. To see how to build, check [here](https://github.com/google/googletest).
+
+## Build
+To build the library and header file.
+```shell
+make build
+```
+
+- The header file `opendal.h` is under `./include` 
+- The library is under `../../target/debug` after building.
+
+To clean the build results.
+```shell
+make clean
+```
+
+## Test
+To build and run the tests. (Note that you need to install GTest)
+```shell
+make test
+```
+
+```text
+[==========] Running 5 tests from 1 test suite.
+[----------] Global test environment set-up.
+[----------] 5 tests from OpendalBddTest
+[ RUN      ] OpendalBddTest.Write
+[       OK ] OpendalBddTest.Write (4 ms)
+[ RUN      ] OpendalBddTest.Exist
+[       OK ] OpendalBddTest.Exist (0 ms)
+[ RUN      ] OpendalBddTest.EntryMode
+[       OK ] OpendalBddTest.EntryMode (0 ms)
+[ RUN      ] OpendalBddTest.ContentLength
+[       OK ] OpendalBddTest.ContentLength (0 ms)
+[ RUN      ] OpendalBddTest.Read
+[       OK ] OpendalBddTest.Read (0 ms)
+[----------] 5 tests from OpendalBddTest (4 ms total)
+
+[----------] Global test environment tear-down
+[==========] 5 tests from 1 test suite ran. (4 ms total)
+[  PASSED  ] 5 tests.
+```
+
+## Docs
+The OpenDAL C binding currently do not have an official documentation online(Open to contribution).
+
+If you want to know more about the OpenDAL C binding, you may:
+- Have a look at the comments inline with Rust codes under the `./src` or the comments in `opendal.h`
+- Have a look at the comments of the Rust core
+- Have a look at the test files, they contain compact and simple usage of OpenDAL C binding
+
+## Misc
+- We use `cbindgen` to generate the header file and library. If you want to contribute to the C binding core code, you may see the official [documentation](https://github.com/mozilla/cbindgen/blob/master/docs.md) of `cbindgen`.
+- We have github action that shows how to setup the building and testing environment, if you are still not clear how to setup your environment, you may see [this github action](../../.github/workflows/bindings_c.yml).