Added iterators section in README

[enrichman]

Fix: #3209

As suggested in the issue, this PR adds in the README some information about how to use iterators with the enrichman/gh-iter package, with a small code snippet.

The feature is still experimental, because it is not available in the standard library yet (Go 1.23 is planned for August 2024).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.92%. Comparing base (2b8c7fa) to head (b0a1ca3).
Report is 83 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3212      +/-   ##
==========================================
- Coverage   97.72%   92.92%   -4.80%     
==========================================
  Files         153      171      +18     
  Lines       13390    11582    -1808     
==========================================
- Hits        13085    10763    -2322     
- Misses        215      726     +511     
- Partials       90       93       +3     

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[gmlewis]

Excellent! Thank you, @enrichman !
LGTM.
Merging.