Add DOI role and reference to Zinke DNB method

[djhoese]

This is the ground work for #500, but I wouldn't say it closes it. I would say this is fully documented, but the role is really not fully documented. It doesn't even exist outside of scipy and the other projects that have copied it from a gist someone made a while ago. The module is copied in to the sphinx doc source. I was able to use it nicely on the NCC DNB compositor for Stephan Zinke's paper.

• Fully documented

[coveralls]

Coverage decreased (-0.02%) to 74.067% when pulling 1f7d9bf on djhoese:doc-references into f842a46 on pytroll:master.

[codecov]

Codecov Report

Merging #501 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #501   +/-   ##
=======================================
  Coverage   74.08%   74.08%           
=======================================
  Files         137      137           
  Lines       18220    18220           
=======================================
  Hits        13499    13499           
  Misses       4721     4721

Impacted Files	Coverage Δ
satpy/writers/__init__.py	76.19% <ø> (ø)	⬆️
satpy/composites/viirs.py	84.09% <ø> (ø)	⬆️

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