docs: add Developing Tsuru Plugins

[tcarreira]

Documentation about plugins, what they are and how to create new plugins

• what is a tsuru plugin: it is an executable file (script or binary)
  • if binary, it should be compiled for supported os/arch
• what information is passed as ENV from the tsuru-client to the plugin
• publishing a plugin (if platform-dependent binary, should publish a manifest.json)
• recommend tagging the github project with tsuru-plugin

Check how this gets rendered:

• https://github.com/tcarreira/tsuru-client/tree/master-doc-tsuru-plugins#tsuru-plugins
• https://github.com/tcarreira/tsuru-client/blob/master-doc-tsuru-plugins/plugins.md

closes: #192

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9966709) 71.55% compared to head (0d96f07) 71.55%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #215   +/-   ##
=======================================
  Coverage   71.55%   71.55%           
=======================================
  Files          44       44           
  Lines        8298     8298           
=======================================
  Hits         5938     5938           
  Misses       1527     1527           
  Partials      833      833           

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[wpjunior]

thanks @tcarreira