Refined the main doc and removed the Pre-release disclaimer
Refined the main doc and removed the Pre-release disclaimer

ibm-watson-iot/iot-java,sathipal/iot-java,BorisDaich/iot-java,amprasanna/iot-java,ibm-watson-iot/iot-java,amitmangalvedkar/iot-java,ibm-messaging/iot-java