Add photosAddOnly setup for iOS to Podfile in README

[c15yi]

Replace this paragraph with a short description of what issue this pull request (PR) solves and provide a description of the change. Consider including before/after screenshots.

The to be commented out comment for only adding photos to the library under iOS (NSPhotoLibraryAddUsageDescription) was missing in the README.

List at least one fixed issue.
#980

Pre-launch Checklist

• I made sure the project builds.
• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is does not need version changes.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I rebased onto main.
• I added new tests to check the change I am making, or this PR does not need tests.
• I made sure all existing and new tests are passing.
• I ran dart format . and committed any changes.
• I ran flutter analyze and fixed any errors.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (978dbeb) 71.42% compared to head (9a06ce1) 71.42%.
Report is 7 commits behind head on main.

❗ Current head 9a06ce1 differs from pull request most recent head 5129a6d. Consider uploading reports for the commit 5129a6d to get more accurate results

Additional details and impacted files

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  Coverage   71.42%   71.42%           
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  Files           1        1           
  Lines          35       35           
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  Hits           25       25           
  Misses         10       10           

Flag	Coverage Δ
unittests	71.42% <ø> (ø)

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[TimHoogstrate]

Dear @ChristianNavolskyi, Thanks for your input. This is a really good find. Could you update the CHANGELOG.md to add a description of the change and increase the version patch number? The change looks great. Thanks

[mvanbeusekom]

LGTM, with some minor formatting changes that I will quickly apply.