doc: add link to contributing and licence file

[rubiin]

Removes the licence text . Instead links to the license file.
Also add a short contributing section and links to contributing markdown file

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)
• References provided in PR (where applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (c25b98f) to head (347e4d2).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2413   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          109       109           
  Lines         2489      2489           
  Branches       628       628           
=========================================
  Hits          2489      2489           

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