Add references for R2 and LBFGS in readme #236

[hpmouton]

This pull request addresses issue #236 by adding references for the R2 and LBFGS implementations in the project's readme.

Changes Made:

• Added references for the R2 implementation in the readme.
• Included references for the LBFGS implementation in the readme.

Related Issue:
Closes #236

[tmigot]

Thanks @hpmouton for the suggestion! I really like the idea.

@dpo or @d-monnet Would you have some time to check if that's the "right" references?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.48%. Comparing base (bd12646) to head (d24ec90).

❗ Current head d24ec90 differs from pull request most recent head a228529. Consider uploading reports for the commit a228529 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #259   +/-   ##
=======================================
  Coverage   88.48%   88.48%           
=======================================
  Files           7        7           
  Lines        1025     1025           
=======================================
  Hits          907      907           
  Misses        118      118           

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[github-actions]

Package name	latest	stable
FletcherPenaltySolver.jl
Percival.jl

[d-monnet]

Thanks @hpmouton for the suggestion! I really like the idea.

@dpo or @d-monnet Would you have some time to check if that's the "right" references?

As far as I know, there's no reference for R2 itself. I'm not sure our paper with D. Orban is the most relevant reference, I'll let @dpo decide what reference is the most appropriate.

[dpo]

Thank you! Just a minor final comment.

[dpo]

Thank you!