chore: add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+## Dependencies
+
+The following are used in the development of Lua-CSharp:
+
+- [Visual Studio Code](https://code.visualstudio.com/) / [Rider](https://www.jetbrains.com/rider/)
+- [Unity Editor](https://unity3d.com/unity/editor) (optional)
+- .NET SDK and runtimes
+- [CSharpier](https://csharpier.com/) (optional, C# code formatter)
+
+## Build
+
+```sh
+dotnet build Lua.slnx -c Release
+```
+
+If you only want to build the NuGet package project:
+
+```sh
+dotnet build src/Lua/Lua.csproj -c Release
+```
+
+## Run Tests
+
+```sh
+dotnet test tests/Lua.Tests/Lua.Tests.csproj -c Release
+```
+
+The test project includes a set of `.lua` conformance-style tests inherited from the official Lua repository. Most of them are expected to pass, but a few known cases are currently marked with the `ExpectedFailure` category in the test suite.
+
+To run the test project while excluding those known failures:
+
+```sh
+dotnet test tests/Lua.Tests/Lua.Tests.csproj -c Release --filter "TestCategory!=ExpectedFailure"
+```
+
+This is the same filter used in GitHub Actions when packing and publishing the NuGet package.
+
+## Unity
+
+The extension package for Unity is located in the `Assets/Lua.Unity` directory within the Unity project, which is located in `src/Lua.Unity`.
+
+The Unity package does not include the source code of the core package. During development, a local package containing the debug DLL is added as `com.nuskey8.lua.unity.internal`.
+
+## Publishing Packages
+
+Publishing to NuGet is handled by a workflow via GitHub Actions. This will run whenever you push a `v.*.*.*` tag.
+
+Before this runs, make sure that the versions of `Directory.Build.props` and the `package.json` file inside your Unity project match. Otherwise, the CI will fail.