Add note in readme.md that lib can not work with ublox M6 modules

[cyberman54]

It should maybe mentioned in the readme.md, that this library is getting the positioning data from UBX message UBX-NAV-PVT, and thus can NOT work with older ublox M6 modules, which do not send this UBX message.

[PaulZC]

Hi Verkehrsrot (@cyberman54),

OK. Fair enough. Added here.

Best wishes,
Paul