Update README

[sfreemire]

Add this to the bulleted list "The MAP-T solution is enabled by:"

• Providing all devices in the local MAP-T domain with the DMR (normally via DHCPv6)

Consider adding DHCP option codes to the elements that use DHCPv6.

Consider a paragraph explaining the PySimpleGUI issue - that they went closed source for all future updates. (Done)

Consider explaining use of individual functions, controls, and displays.