Create CONTRIBUTING.md

CONTRIBUTING.md

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+Hiddify uses [Go](https://go.dev), make sure that you have the correct version installed before starting development. You can use the following commands to check your installed version:
+
+
+```shell
+$ go version
+
+# example response
+go version go1.21.1 darwin/arm64
+```
+
+### Working with the Go Code
+
+> if you're not interested in building/contributing to the Go code, you can skip this section
+
+The Go code for Hiddify Next can be found in the `libcore` folder, as a [git submodule](https://git-scm.com/book/en/v2/Git-Tools-Submodules) and in [core repository](https://github.com/hiddify/hiddify-next-core). The entrypoints for the desktop version are available in the [`libcore/custom`](https://github.com/hiddify/hiddify-next-core/tree/main/custom) folder and for the mobile version they can be found in the [`libcore/mobile`](https://github.com/hiddify/hiddify-next-core/tree/main/mobile) folder.
+
+For the desktop version, we have to compile the Go code into a C shared library. We are providing a Makefile to generate the C shared libraries for all operating systems. The following Make commands will build libcore and copy the resulting output in [`libcore/bin`](https://github.com/hiddify/hiddify-next-core/tree/main/bin):
+
+- `make windows-amd64`
+- `make linux-amd64`
+- `make macos-universal`
+
+For the mobile version, we are using the [`gomobile`](https://github.com/golang/go/wiki/Mobile) tools. The following Make commands will build libcore for Android and iOS and copy the resulting output in [`libcore/bin`](https://github.com/hiddify/hiddify-next-core/tree/main/bin):
+
+- `make android`
+- `make ios`