Explicitly explain each output (aka MOM6 input) file

[ashjbarnes]

cc pointed out (#143 ) that we ought to explain out inputs / outputs. I think we've done a good job of the inputs but not the outputs! Since there's not a lot of MOM6 documentation on all the input files required, it would be good to explain what we understand about them in our package somewhere to help newcomers troubleshoot their setups.