Update toolkit information on project page to remove Kafka and MQTT

[natashadsilva]

Minor issue - on the main project page, the tookit information says: This toolkit is focused on interacting with popular messaging systems such as Kafka, JMS, XMS, and MQTT
Since there are now separate toolkits for MQTT and Kafka, please edit this tagline to remove Kafka and MQTT to avoid confusion.

[schubon]

Kafka, RabbitMQ, and MQTT are deprecated only yet. I'm afraid when we remove these toolkits from the project page / readme.md, this will frighten customers of a canceled feature w/o due warning period. The deprecated hints shall appear in the JavaDoc, log printouts, and on the icons. I think we shall remove hints to this functionality only when we remove the respective parts from the toolkit.