Document netcdf feature support

[pp-mo]

The limitations of the of Netcdf4 support / feature coverage.
CF. NUG

I.E. mostly excluding the extended "User defined" NC4 data types:

• "Enum"
• "Opaque"
• "Compound" -- structure-like
• "VariableLength" -- notably, including variable-length-strings

Note on string/character data handling

• strings presented as arrays of "S1"
• no unicode support for character data
  • though there is for names of components (variables, groups, dimensions, attributes)
  • ... ?? and in string attributes ??

Also a note on groups:

• ncdata supports them
• iris/xarray (mostly) don't

[pp-mo]

Done #109