Add a README.md for solidus_legacy_promotions

[mamhoff]

This is mostly taken from the edgeguides repo. It will need to be updated once all the models are moved, but at least we have something.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.72%. Comparing base (6684502) to head (13f9346).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5785   +/-   ##
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  Coverage   88.72%   88.72%           
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  Files         712      712           
  Lines       16914    16914           
=======================================
  Hits        15007    15007           
  Misses       1907     1907           

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[tvdeyen]

Amazing!