Add CONTRIBUTING.md (#154)

* Add CONTRIBUTING.md

* update

* update

Author: trask

CONTRIBUTING.md

@@ -0,0 +1,24 @@
+# Contributing
+
+Welcome to the OpenTelemetry Semantic Convention Java repository!
+
+## Building
+
+Java 17 or higher is required to build the projects in this repository
+(the built artifacts can be used on Java 8 or higher).
+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).