docs: add information about spotless for code formatting (#2622)

Signed-off-by: James Rich <2199651+jamesarich@users.noreply.github.com>

CONTRIBUTING.md

@@ -14,6 +14,8 @@ Thank you for your interest in contributing to Meshtastic-Android! We welcome co
 
 - Follow the [Kotlin Coding Conventions](https://kotlinlang.org/docs/coding-conventions.html) for Kotlin code.
 - Use Android Studio's default formatting settings.
+- We use [spotless](https://github.com/diffplug/spotless) for automated code formatting. You can run `./gradlew spotlessApply` to format your code automatically.
+  - You can also run `./gradlew spotlessInstallGitPrePushHook --no-configuration-cache` to install a pre-push Git hook that will run a `spotlessCheck`.
 - Write clear, descriptive variable and function names.
 - Add comments where necessary, especially for complex logic.
 - Keep methods and classes focused and concise.