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.