CORE-4667: Correcting JSON keys for consistency and updating README

README now suggests insertion of milliseconds since epoch for time/date fields.
All occurrences of **activation-date** and **deactivation-date** have been replaced by **activation_date** and **deactivation_date** respectively. 
In other words, the dashes in the previously mentioned JSON keys have been replaced by underscores. This is for consistency across the endpoints in this module.