docs(webex-core): add service referenc to readme
docs(webex-core): add service referenc to readme

Add a reference to services within the readme to identify that
it is now a part of webex-core.

adamweeks/spark-js-sdk,aimeex3/spark-js-sdk,mwegman/spark-js-sdk,adamweeks/spark-js-sdk,adamweeks/spark-js-sdk,aimeex3/spark-js-sdk,mwegman/spark-js-sdk,aimeex3/spark-js-sdk,mwegman/spark-js-sdk