Update README.md re. vgrid

[aekiss]

See #33

[micaeljtoliveira]

@aekiss It should be easy to add an assertion in the Fortran code to check if the number of values is odd, as that should always be the case for MOM5 vgrids (2n+1 values for n levels). It wouldn't complain about MOM6 vgrids where n+1 is odd, but it's better than nothing... What do you think?

[aekiss]

Yep, might as well, but with a comment to explain it will only catch this problem half the time.
It won't help with the 51 vs 101 case we discussed yesterday, but would catch 76 vs 151 which will be the new default at all resolutions.

[micaeljtoliveira]

Merging this. Will implement the assertion in the Fortran code in a separate MR.