Improvement of rs41/README.md for inverted signals (as in the repo)

[maehw]

Hi!

Thanks for sharing your knowledge and code. It may not be worth a pull/merge request, but I had some difficulties getting started.

It seems that both recordings in rs41/wav are inverted. The rs41/README.md mentions inverted signals and could be expanded by the following example to be used with the wav file from the repository out of the box:

$ sox ./wav/20140717_402MHz.wav -t wav - lowpass 2800 2>/dev/null | ./rs41ptu --ecc --crc -vx -i

Cheers