Add thread-safety note for NetCDF4

[danlooo]

see #450

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.38%. Comparing base (775ce9c) to head (c9f55a2).
Report is 38 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #451      +/-   ##
==========================================
+ Coverage   68.40%   69.38%   +0.98%     
==========================================
  Files          12       12              
  Lines        1807     1813       +6     
==========================================
+ Hits         1236     1258      +22     
+ Misses        571      555      -16     

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[coveralls]

Pull Request Test Coverage Report for Build 11365186998

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage remained the same at 69.464%

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Files with Coverage Reduction	New Missed Lines	%
src/DAT/DAT.jl	1	76.05%

Totals
Change from base Build 10982138529:	0.0%
Covered Lines:	1258
Relevant Lines:	1811

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💛 - Coveralls

[felixcremer]

Could we make this documentation a bit more explicit and show on an example how to use these locks for threadsafe opening a NetCDF?