docs(resource-detector-github): update readme for github resource detector

[maryliag]

Which problem is this PR solving?

• Part Of improve resource detector documentation #2055

Short description of the changes

• Update README of GitHub Resource Detector to clarify the source of the data

[codecov]

Codecov Report

Merging #2064 (bf595ec) into main (dfb2dff) will decrease coverage by 0.34%.
Report is 46 commits behind head on main.
The diff coverage is n/a.

❗ Current head bf595ec differs from pull request most recent head b2d8b15. Consider uploading reports for the commit b2d8b15 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2064      +/-   ##
==========================================
- Coverage   90.97%   90.63%   -0.34%     
==========================================
  Files         146      146              
  Lines        7492     7488       -4     
  Branches     1502     1494       -8     
==========================================
- Hits         6816     6787      -29     
- Misses        676      701      +25     

see 8 files with indirect coverage changes