Basic SDF documentation

[grantfirl]

(Transferred from https://github.com/NCAR/NEMSfv3gfs/issues/97)

Now that SDFs reside in host model repositories and given their propensity to multiply during testing, I think that it would be a good idea to provide some kind of high level documentation on what the suites represent more than just relying on the name of the file. Are comments allowable in the SDFs? Failing that (or in addition), could there be one text file in the /ccpp/suites directory that lists the suites in the repo and what they are and how they differ? Right now, for folks outside of GMTB, I imagine that it is pretty difficult to get a feel for what the suites are and how they differ -- basically they have to read through the SDFs line by line and compare.

From @climbfuji:
Comments are allowed:

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Sounds like a very good idea for the UFS release.