Standardize contributing section (#731)

Author: trask

README.md

@@ -72,13 +72,17 @@ of them assume you have docker running on your local machine.
       zipkin backend using the OpenTelemetry API.
     - Note: this example requires having docker installed to run the example.
 
-## Maintainers
+## Contributing
+
+See [CONTRIBUTING.md](CONTRIBUTING.md).
+
+### Maintainers
 
 - [OpenTelemetry Java Maintainers](https://github.com/open-telemetry/opentelemetry-java#maintainers)
 - [OpenTelemetry Java Instrumentation Maintainers](https://github.com/open-telemetry/opentelemetry-java-instrumentation#maintainers)
 - [OpenTelemetry Java Contrib Maintainers](https://github.com/open-telemetry/opentelemetry-java-contrib#maintainers)
 
-## Approvers
+### Approvers
 
 - [OpenTelemetry Java Approvers](https://github.com/open-telemetry/opentelemetry-java#approvers)
 - [OpenTelemetry Java Instrumentation Approvers](https://github.com/open-telemetry/opentelemetry-java-instrumentation#approvers)