Add a first version on how to write a custom reader

[mraspaud]

• Fully documented

[djhoese]

Should this maybe be added to the developer's guide section? Or a separate page as a whole? This main readers page could still reference the other page. Thoughts?

[coveralls]

Coverage increased (+0.2%) to 74.13% when pulling c490b5b on mraspaud:doc-custom-reader into 5dd28ea on pytroll:master.

[codecov]

Codecov Report

Merging #489 into master will increase coverage by 0.19%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #489      +/-   ##
==========================================
+ Coverage   73.93%   74.13%   +0.19%     
==========================================
  Files         137      137              
  Lines       18222    18257      +35     
==========================================
+ Hits        13472    13534      +62     
+ Misses       4750     4723      -27

Impacted Files	Coverage Δ
satpy/config.py	46.66% <0%> (-24.49%)	⬇️
satpy/scene.py	84.03% <0%> (-0.43%)	⬇️
satpy/tests/test_scene.py	99.32% <0%> (-0.01%)	⬇️
satpy/composites/viirs.py	84.09% <0%> (ø)	⬆️
satpy/tests/test_enhancements.py	98.55% <0%> (ø)	⬆️
satpy/readers/hrit_msg.py	44.63% <0%> (ø)	⬆️
satpy/writers/geotiff.py	43.51% <0%> (ø)	⬆️
satpy/readers/abi_l1b.py	96.66% <0%> (+0.02%)	⬆️
satpy/readers/iasi_l2.py	98.8% <0%> (+0.02%)	⬆️
satpy/tests/test_writers.py	97.31% <0%> (+0.11%)	⬆️
... and 9 more

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