opentelemetry-php/code-of-conduct.md

#Code Of Conduct
opentelemetry-php follows the code of conduct that the rest of the OpenTelemetry [community](https://github.com/open-telemetry/community/blob/master/code-of-conduct.md) follows, the [CNCF Code of Conduct](https://github.com/cncf/foundation/blob/master/code-of-conduct.md)