Mention the community .NET Standard port in the README (#5)

[pscheid92]

Adds a pointer to @davidmatson's AgeSharpNetStandard port for .NET Framework 4.8 / .NET Standard 2.0 users, as requested in #5.

Framed honestly: third-party, not maintained or audited by this project, and a point-in-time port that may lag behind upstream (including security fixes) — so readers know to vet it themselves rather than treating it as an official variant.

Fixes #5

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.50%. Comparing base (7f74d10) to head (a649b5d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #12      +/-   ##
==========================================
+ Coverage   91.46%   91.50%   +0.04%     
==========================================
  Files          37       37              
  Lines        2320     2320              
  Branches      304      304              
==========================================
+ Hits         2122     2123       +1     
+ Misses        141      140       -1     
  Partials       57       57              

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