chore: add LICENSE file

[ggulgulia]

added license file based on APACHE 2.0

Closes #71

Summary

Added license file
copied it from ign-physics license file

[chapulina]

Thanks! Could you sign your commit for the DCO checker? Thanks

[ggulgulia]

@chapulina: fixed the previous commit. Thanks for the hint

[codecov]

Codecov Report

Merging #72 (9da5b41) into ign-plugin1 (adced8d) will not change coverage.
The diff coverage is n/a.

❗ Current head 9da5b41 differs from pull request most recent head 6ffd148. Consider uploading reports for the commit 6ffd148 to get more accurate results

@@             Coverage Diff              @@
##           ign-plugin1      #72   +/-   ##
============================================
  Coverage        99.82%   99.82%           
============================================
  Files               15       15           
  Lines              584      584           
============================================
  Hits               583      583           
  Misses               1        1           

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