Add info which unit has to be used to calculate, report, store and archive the salinity data.

[soerenthomsen]

We have to add this information somewhere and make sure we align well with the data management team, ARGO, etc

This should also mention that the format and vocabulary of OG should be used i.e. linking to the OG1.0 document in the OceanGliders Data Management Task Team repo.