[Event Hubs Client] Enhance ReadMe

[jsquire]

Update the client library ReadMe to adapt to the guidelines for format and content where reasonable to do so.

While the ideal goal is conform to the template, the goal is not 100% compliance. Some content should be left as it was written in the standalone repository in order to keep to the scope of work of adopting the client library source into the central repository, as opposed to moving all Service Bus-related artifacts.

[jsquire]

Work is believe to be complete; to be included as part of the work for [Event Hubs] Migrate Client Code into Central Repository