WIP - Document the way to write reader and writer plugins

[chfw]

With your PR, here is a check list:

• Has test cases written?
• Has all code lines tested?
• Has make format been run?
• Please update CHANGELOG.yml(not CHANGELOG.rst)
• Passes all Travis CI builds
• Has fair amount of documentation if your change is complex
• Agree on NEW BSD License for your contribution

[codecov-commenter]

Codecov Report

Merging #91 into dev will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev      #91   +/-   ##
=======================================
  Coverage   97.19%   97.20%           
=======================================
  Files          52       52           
  Lines        3282     3292   +10     
=======================================
+ Hits         3190     3200   +10     
  Misses         92       92           

Impacted Files	Coverage Δ
pyexcel_io/plugin_api/abstract_reader.py	100.00% <100.00%> (ø)
pyexcel_io/plugin_api/abstract_sheet.py	100.00% <100.00%> (ø)
tests/test_plugin_api.py	100.00% <100.00%> (ø)

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