Add CONTRIBUTING.md (#596)

CONTRIBUTING.md

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+# Contributing
+
+Welcome to the OpenTelemetry Java Examples repository!
+
+## Building
+
+Java 17 or higher is required to build the projects in this repository.
+To check your Java version, run:
+
+```bash
+java -version
+```
+
+To build the project, run:
+
+```bash
+./gradlew assemble
+```
+
+## Style guide
+
+This repository follows the OpenTelemetry Java
+repository's [style guide](https://github.com/open-telemetry/opentelemetry-java/blob/main/CONTRIBUTING.md#style-guideline).
+
+## Gradle conventions
+
+- Use kotlin instead of groovy
+- Plugin versions should be specified in `settings.gradle.kts`, not in individual modules
+- All modules use `plugins { id("otel.java-conventions") }`