Clarify the role of the DATA response in fastboot

The README is updated to explicitly state that the DATA response packet
in fastboot does not contain any actual data, but rather indicates that
the data transfer will begin with the next packet.

Bug: none
Test: none
Flag: EXEMPT not required, readme change
Change-Id: Ie92859f1f6e1c925ef4d58186ff6b35584dc6a10