Update outdated link in README.md

[Xtrah]

Why are these changes needed?

The link to GitHub Docs is outdated and currently directs to GitHub Support and not the relevant GitHub Docs. It is now updated to use GitHub-owned opensource.guide on contributing to open source projects.

Related issue number

N/A

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5b217c9) 34.24% compared to head (bea5a6a) 32.73%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1541      +/-   ##
==========================================
- Coverage   34.24%   32.73%   -1.52%     
==========================================
  Files          42       42              
  Lines        5098     5098              
  Branches     1165     1165              
==========================================
- Hits         1746     1669      -77     
- Misses       3209     3291      +82     
+ Partials      143      138       -5     

Flag	Coverage Δ
unittests	32.73% <ø> (-1.52%)	⬇️

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