Add LICENSE.md

[bvobart]

GPL license seems to me the most appropriate for mllint

[codecov-commenter]

Codecov Report

Merging #17 (9ed90ac) into main (f59c814) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #17   +/-   ##
=======================================
  Coverage   86.19%   86.19%           
=======================================
  Files          48       48           
  Lines         884      884           
=======================================
  Hits          762      762           
  Misses         84       84           
  Partials       38       38           

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