Add new DESCRIPTION mdoc variable with description setting value

[alejandrohdezma]

No description provided.

[codecov]

Codecov Report

Merging #159 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #159   +/-   ##
=======================================
  Coverage   82.60%   82.60%           
=======================================
  Files          21       21           
  Lines         184      184           
  Branches        1        1           
=======================================
  Hits          152      152           
  Misses         32       32           

Impacted Files	Coverage Δ
...rohdezma/sbt/github/mdoc/SbtGithubMdocPlugin.scala	0.00% <ø> (ø)

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