Add submodule to Git

Hi there,

I have succesfully integrated Google Wavenet to Rhasspy 2.5.
In my own fork, I have added a submodule rhasspy-tts-wavenet-hermes and also change rhasspy-supervisor and rhasspy-server-hermes.

I am not confident on how to add the submodule to rhasspy from my fork, can that be done with a PR?
Never used submodules :smiley: