Update bluetooth/tester README for how the requirements are to run this test

[galak]

tests/bluetooth/tester README isn't really clear about what hardware and software requirements are needed to run this test. It appears that one needs PTS from bluetooth sig and some associated HW for this to run, but not clear.

[Vudentz]

@rmstoi Can you update the README, we should probably mention what are the required environment and the expected UART ports as well.