Change global attributes

[fenclmar]

Jafet and Remco a few questions/suggestions concerning the white paper left.
• netCDF_global_attributes.adoc:

1. Add “version” as a recommended attribute as well. This is very useful if the dataset is updated for whatever reason (may be other reasons than changes of version in the “source”). This would be easier to check than “history”.
2. we suggest only using “licence” instead of “licence restrictions” since the licence may not imply any restrictions at all. Of course you could say 'None'.
3. under “comment” add also “e.g. contact"