Add link to project roadmap to the README

[benjaminwil]

Summary

Per the discussion on #6229, it is nice to improve the discoverability of our existing project roadmap.

Checklist

• I agree that my PR will be published under the same license as Solidus.
• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have used clear, explanatory commit messages.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.87%. Comparing base (e6a57c3) to head (2420814).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6284   +/-   ##
=======================================
  Coverage   88.87%   88.87%           
=======================================
  Files         858      858           
  Lines       18413    18413           
=======================================
  Hits        16364    16364           
  Misses       2049     2049           

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[tvdeyen]

Thanks