[ci skip] Add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing guidelines for the project
+
+## Setting up a Workspace
+ViaFabricPlus uses Gradle, to make sure that it is installed properly you can check [Gradle's website](https://gradle.org/install/).
+1. Clone the repository using `git clone https://github.com/ViaVersion/ViaForge`.
+2. CD into the local repository.
+3. Run `./gradlew build`.
+4. Open the folder as a Gradle project in your preferred IDE.
+5. Run the mod.
+
+## Add a new feature or fix a bug
+1. Create a new branch for your feature/bugfix (e.g. `feature/fix-xyz` or `fix/fix-xyz`)
+2. Implement your feature/bugfix and make sure it works correctly
+3. Clean your code and make sure it is readable and understandable (e.g. use proper variable names)
+4. Use the Google java code style (https://google.github.io/styleguide/javaguide.html) and format your code accordingly
+5. Create a pull request and wait for it to be reviewed and merged.
+6. You're done, congrats!
+
+## Add support for a new Minecraft version
+1. Create a new branch for the new version (e.g. `update/1.20.6`)
+2. Create a new `viaforge-mc<version>` folder in the root directory of the project
+3. Add a `gradle.properties` file with the `forge_version` set (Available at https://files.minecraftforge.net/)
+4. Register it inside the `settings.gradle` file
+5. Copy the code from the previous version and update it accordingly
+
+### Build logic
+The `build.gradle` file contains the shared build code for all submodules. Build code for only specific versions
+of the game can be wrapped with checks using the `versionId` integer which will be parsed from the `forge_version`.
+
+An example would be:
+````groovy
+if (versionId >= 1_13_2) {
+    // We don't need to package mixins into Forge 1.13+ jars, since Forge already has it
+    exclude("org/spongepowered/**")
+}
+````
+
+### Notes
+Shared source code is inside the ``src/main/java`` root folder, while version-specific code is inside the ``/viaforge-mc<version>`` folders.