docs: Adds FOSSA badge for license scan

[frgfm]

This PR adds a badge for license scan of all dependencies, the goal being to have both:

• transparency over license compliance
• detecting changes of this status as the project evolves

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8b7f164) 82.59% compared to head (97ef752) 82.59%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #54   +/-   ##
=======================================
  Coverage   82.59%   82.59%           
=======================================
  Files          29       29           
  Lines         971      971           
=======================================
  Hits          802      802           
  Misses        169      169           

Flag	Coverage Δ
unittests	82.59% <ø> (ø)

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